It's not about S3/Dropbox/etc as much as it is about decoupling the service from the storage mechanisms. Once you do that then a whole lot of opportunities present themselves which otherwise didn't exist.

So yes, Amazon may ditch S3 but if the file system is decoupled from the service you're using and there are adapters for alternatives (which there are) then it's trivial to migrate without any "loss of service".

That's the worst case scenario...which isn't too bad.

Note, there's local file system and mysql adapters as well if you don't trust Amazon/Dropbox.