In no way is St. Optatus suggesting that Rome is an eternal chair that has to be maintained. He is maintaining that Rome is the chair of the chief of the Apostles, who was given the title "Cephas". This title is the chair upon which all Apostles base their Church.
Suppose, we have a Donatist in Egypt. An Egyptian bishop can easily say:
You cannot then deny that you do know that upon Peter first, who sent Mark to the City of Alexandria, was bestowed the Episcopal Chair [Cathedra], on which sat Peter, the Head of all the Apostles (for which reason he was called Cephas), that, in this one Chair, unity should be preserved by all, lest the other Apostles might claim--each for himself--separate Chairs, so that he who should set up a second Chair against the unique Chair would already be a schismatic and a sinner. Well then, on the one Chair, which is the first of the Endowments, Peter was the first to sit.
You and I cannot claim this quote means the very city of Rome must be existent for the Church to be the Church. Rome is an accident, not a particularity of the Church. This very seat, the Episcopal Chair, the Cathedra is the binding and loosing, the keys of heaven, granted to every bishop in the universal church. Later, St. Optatus writes, ridiculing the Donatists:
So, of the above-mentioned Endowments, the Cathedra is, as we have said, the first, which we have proved to be ours, through Peter, and which draws to itself the ANGEL ----unless, perchance, you claim him for yourselves, and have him shut up somewhere or other. Send him out if you can, and let him exclude from his communion seven angels, our colleagues in Asia, to whose churches wrote the Apostle John----churches with which you cannot prove that you have any intercourse whatsoever.
Optatus is not the bishop of Rome, but he receives his authority through Peter. Every Church has an angel. The chair of Peter has this angel. If Donatists claim this angel for itself, let them prove that this Angel is separate in communion from the angels of the churches established by St. John.