The reason it won't is because the curves are not compatible. (Ok you could play seed phrase games, but no point). But if it's zapchat then you could absolutely have a zapchat on pubky, you wouldn't have to design a whole new zapchat, keep the signed notes, relays and all. But you would need to go to an Ed curve, so breaking change.

You seem to be treating Pubky as some sort of more general idea, while in fact they are two completely disjoint things. Nostr assumes don’t trust, verify by default but can be loosened. With Pubky, each application needs to invent its own way of verifying and it is discouraged by the design. Nostr relays are inherently simpler than Pubky homeservers. Nostr can also propagate data through a variety of methods, while Pubky makes it hard to not rely on homeservers.