That seems to be the most common fear, but I can't see devs moving their apps off the default app store - that would be a terrible move for sales.

The only apps that are going to be available on third party app stores but not on the official one are going to be apps that would have never been available on iPhone at all in the first place if not for this change.

We already have a real world example of this exact situation with Android, and there's no real store fragmentation there.