I think we've had this question rather frequently. The majority of the board tends to think that Byzantium might do well, while the Sassanids were in shambles anyway and would maybe buy a century or two, but few consider holding out like the ERE did until the 15th century likely for them.
I am not quite so convinced of this. In a recent thread, many people said that even without Islam, Arab conquests may well have happened, even though they'd maybe lack the coherence and longevity and would soon acculturate. I think that is rather likely. So, the ERE is thrown back upon its Anatolian and Balkanic holdings anyway, and I´m not sure they could win North Africa back from whoever holds it in a few centuries' time. As for the Sassanids, that'd be the loss of Mesopotamia, and they might not recover that, either, which still cripples them. But that doesn't mean that Iran's highlands would fall, too - I can see the Zoroastrian state hold out there for many, many centuries, and I think the land which we now call Kurdistan would still remain perenially contended, only now in a three-way fight between Byzantium, Sassanids, and an Arab Mesopotamian state, with the Göktürks or their successors from the North acting as a fourth wildcard maybe.