It looks like more and more applications are settings the minimum iOS level to version 5. This means there are no further updates for iPhone 3G users and if you wipe your phone you may not be able to get the same apps again.
Your only hope of keeping your old application is to ensure you have a backup that you can restore.

This doesn’t apply to all applications from the iTunes store but it’s slowly getting that way.

Its understandable that technology moves on, but how long before your beloved iphone because a simple MP3 player.