Oh, good point! Indeed, the iPhone 3G can't be upgraded to iOS 5, limiting what applications can be used on the device. While Apple does have a system for allowing users on older devices to install the latest compatible version of an app, that only helps if a compatible version exists, and it still works with the application servers. With an increasingly small number of applications available to use bandwidth, it stands to reason that average bandwidth usage per iPhone 3G user would go down over time.

Personal experience doesn't bear that out. Back when I had an iPhone 3G, my most-used app was for radio streaming. I consumed 5gigs in a month easy with that. Don't confuse slower with incapable. The real question is if you can't afford a newer phone, can you still afford the more expensive data plans?

Right.. but as time goes on, an increasing number of those applications will have released mandatory updates that require iOS 5 or later, leaving 3G users unable to continue using them. With a shrinking number of working applications to choose from, it's inevitable that usage will drop.

You can download "the last supported version" if you want, the app store offers that now. And any apps already installed or purchased can be re-installed at a later date.

Right, but that only gives you an old version of the application - it doesn't include a matching version of the online service the application is connecting to. If the service has changed sufficiently, the old application may no longer work properly (or may just get a "too old, please upgrade" message).