"Such is the futility of hypothetical discussions."
Is that so? It is my understanding that hypothetical thinking (including the hypothetical 'questioning' you seem to hold in such low esteem) underlies much of the scientific method itself - testable hypotheses and theories often begin with 'what-if' speculations about some phenomenon. In fact, I often find in my own scientific (financial and economic) explorations that consideration of the boundary conditions, e.g. the point of catastrophic failure, no matter how unlikely we may consider them to be, can yield productive insights for analysis in normal conditions as well. I also note that such analyses are an important part of various engineering disciplines, such as reliability and industrial engineering, and an important tool in business analysis. Of course, as an economist I would naturally be attracted to the 'margins' of thought (note: bad pun alert!). And as we say in finance, "One-in-a-million-year events seem to happen every few years."
I think Ebor raises some excellent points/clarifications of his own in discussing issues of literacy, Bible availability, wine availability, etc. Indeed the worst-case scenario I had in mind was a dark age more severe (or at least more extensive) than any the world has experienced previously (though still not technically 'apocalyptic'). Think of the British dark age period doubled in length, or the dark ages of ancient Greece or Egypt. The fact is that humans have a consistent and remarkable inability to judge catastrophic risk - we either assign sich risks a probability of zero (the "It Can't Happen to Me Syndrom"), or we assign them a probability of one (the "Chicken Little Syndrom", more recently renamed the "Global Warming Self-Flagellation Syndrom"). As we again say in finance, the distribution of possible events is 'fat-tailed', meaning that extremes are more likely than with a normal distribution (but still not inevitable). We are constantly surprised and caught off-guard by sudden changes (like for example 9-11 or the collapse of the Soviet Union), and yet we continue to blithely assume that what prevails today will likely prevail in the future, even the distant future. It is a useful heuristic for normal daily life, but not for planning or thinking about the long-term. This is another use for hypothetical questions and worst-case thinking - they help us go beyond our usual prejudices to consider possibilities that we tend to disgard because the probability is felt to be very very small. And yet again, those who have made a study of risk know that there is a world of difference between a probability of "nearly zero" and a probability of "zero".
Think of these historical examples of dark ages catastrophes extended to cover the globe for a period of five or more centuries. In that time much of the 'old world' we currently live in would be lost, forgotten, or just passed by. Imagining what societies emerging from such an extended catastrophe would look like is probably pointless (but might make for a fun novel) - we just know that they would be radically different from the world we live in now. And yet Orthodox and other traditionalist Christians claim that their faith would remain a constant through such changes, since the faith is based on absolutes that are timeless and eternal. In fact, this claim is usually at the core of the value proposition Orthodox make in defending the uniqueness of their faith. Given all this, I think consideration of the basis of this claim is extremely relevant, indeed essential. By subjecting the claim to various tests, both historical and hypothetical, I would hope to gain a better understanding of the institutional factors that make Orthodoxy robust. Perhaps then one can understand exactly why it is proper to have strong faith in the biblical promise about the gates of hell not prevailing, yes? Ultimately, as Ebor rightly points out, this sort of analysis, while not definitive to be sure, can certainly help delineate core from peripheral concerns. For example, while Greek festivals and other ethnic aspects of Church life are nice, do they hold up as core practices under our hypothetical? Clearly not. On the other hand, does the institution of monasticism? Indeed it does, and like with post-Roman Britian and Europe may arguably be THE critical component for maintaining the faith through a future catastrophe.
These then are my thoughts in very broad outline. Now to turn to specifics. I believe the following institutional structures all contribute to the robustness of Orthodoxy in the face of a changing world, allowing Orthodox to claim with much legitimacy that theirs is a timeless faith:
1. Monasticism as the engine for the creation of saints and maintenance of legitimate and confirmable spiritual method. In my limited studies this seems to be a truly critical factor. Historically speaking, monastics have played an important role at times in correcting for excesses or doctrinal drift. Furthermore, as the examples of Britain and Ireland show, monasticism played an important role in re-Christianizing areas that had collapsed into pre-Christian modes, and in preserving much knowledge.
2. The Orthodox notion of theology being experiential rather than intellectual in nature. In other words, the theologian is someone who has "been there" and is passing on direct knowledge of God (His uncreated energies at least) rather than someone who is just a believing philosopher or academic. I think this is an important component since, as with science, there is a promise that through the application of a specific method (in this case a spiritual method) anyone may achieve a state of illumination. As theology is based in experience, it is personally testable in a manner similar to the way scientific claims are testable, even though spiritual claims necessarily lack the objectively measurable character of most scientific claims. Furthermore, by emphasizing experience (i.e., direct revelation) over indirect revelation (i.e., talking about what was revealed to someone else) Orthodox are less reliant on a single source such as the Bible.
3. As many Orthodox like to point out, the liturgy is heavily Biblical, so even without regular access to a Bible most Orthodox would still be able to get the essentials. Even if the priesthood along with the laypeople were largely non-literate, the liturgy provides a sound structure for preserving teachings without books. In pre-literate societies ritual was an important means of transmission of knowledge, so any robust Christian tradition should have a strong emphasis on ritual and sacraments.
4. Icons are an important means of preserving the Biblical stories in a format that doesn't require literacy.
5. Historical depth. I think it fair to say that Orthodox thinking is strongly historical, with its emphasis on the Fathers, the Councils, the Saints, and so forth. This historical knowledge is well diffused among laypeople allowing for much to be preserved even in a non-literate world.
All of these factors deal with the preservation and transmission of institutional knowledge, which I think the Orthodox excel at compared to most other Christian sects. The multiple means of preserving knowledge - monasticism, the liturgy and sacraments, icons, the Bible, etc. - all covered under the heading "Tradition", create significant redundancy and thus robustness.
In addition to institutional knowledge, the other aspect that should be considered is ecclesiastical robustness. How well could the Church structure withstand the loss of mobility and communication? How well would the Church be able to withstand societal chaos and changing political authorities? I have a few general thoughts on factors affecting this as well.
6. Size and Geographic spread. The Orthodox represent the second largest Christian sect worldwide, after the Roman Catholics. In a global catastrophe one would expect size and dispersion to be mitigating factors for risk. While other traditionalist Christian groups, for example the Amish, may have robust means for transmitting knowledge across generations without change, such groups are often small and geographically concentrated. These increase the risk that they may not survive at all in the upheavals imagined.
7. A networked hierarchical structure based on conciliarity. Orthodox ecclesiology seems to me to be a combination of top-down and bottom-up organization. It is neither too democratic nor is it too centralized. In a dark age world where leadership is pushed down to the local level, but where strong and charismatic leadership is definitely needed, the Orthodox approach seems likely to breed good local leaders (i.e., bishops) at need in a crisis - at least in theory. And yet the conciliar tradition of obedience to synods, councils, and apostolic succession should act as a check on strong leaders who might otherwise be tempted to abuse power or promote heresies - again at least in theory. Orthodox are already used to a certain organizational flexibility even as they also embrace a healthy respect for authority - these qualities lend themselves to organizational robustness I think.
These then are some of my tentative thoughts on the question I posed. I have some other thoughts but no time right now.