IMO, most of the mentioned services are not in danger if they focus on differentiation:
Dropbox: How long will it take until all Dropbox-enabled apps are upgraded to iCloud and Dropbox integration dropped? Differentiation for Dropbox: Ease of use across MULTIPLE platforms.
Camera+: Has a lot more to offer than just crop and rotate. Focus on best post-processing and sharing, turn it into a universal app. Camera+ can read the photo roll, so it can be used to optimize photos taken taken with the new camera app as well.
Instapaper, ReadItLater: Focus on social features, cross platform sharing.
Flipboard: Nothing in iOS5 seems to come near a "personal magazine". Newsstand? Not really something that comes near a 'personal social magazine'.
To-do apps: What about cross-platform, collaboration, reusable lists? Lots of room to differentiate.
In addition, all these developers either have made money through direct sales or advertising. Money they made by using an ecosystem built by Apple. They may make less money in the future if they can't differentiate. But as long as Apple isn't banning e.g. Dropbox-enabled apps, I don't think they are so 'unlucky' souls and nobody is forced to abandon these apps.