Plus, weren't bishops actually originally the only ones called priests, because they performed the Eucharist? It was only later that presbyters too were called priests because they started performing the Eucharist with the consent of their bishop.
To be honest, I'm not sure about this aspect of the history. Yes, I think we can say that it was the "original" practice for a bishop to celebrate the Liturgy surrounded by his presbyters, deacons, and laity, but I'm not sure this proves that presbyters couldn't celebrate it as well: whether they celebrate it independently (e.g., in another building) or in the presence of the bishop himself, they require his blessing, so "permission" is a moot point.
In at least some traditions, it was the practice for the bishop to preside over the Liturgy of the Catechumens (because this was where the word was preached, and preaching/teaching is the charism of the episcopal office) and then to delegate the Liturgy of the Eucharist to a presbyter of his choosing. This was the case in Syriac practice, and to this day the rubrics for a priest's Liturgy in the presence of a bishop bear witness to this practice. So too do the rubrics for ordinations in Byzantine practice, in which deacons, priests, and bishops are ordained precisely at the moment each would be needed in the Liturgy, and not at the same point in the service no matter what the rank.
It's possible that "iereus" was limited to bishops at first, but I don't know that for sure. If so, I would think it had more to do with the fact that they are "full" priests than with an ability to celebrate the Eucharist which may not have been so exclusive.