You are over generalizing. That's only true if "new apps" only support iOS6. "new apps" that choose to support iOS5 will update and download just fine.
The vast majority of apps will still run on iOS5, and likely will for at least a few more updates (based on the developer's decision).
Running the latest and greatest software usually means running the latest and greatest hardware. That's what the Apple platform is geared towards. Software developers are incentivized to keep their apps running on the latest OS, and customers are incentivized to run the latest hardware. For people who care about that, this is a good platform choice.
If you want to use the same phone for 5 years, pick a different platform. Granted, you probably won't have the latest and greatest of anything. Not the latest and greatest hardware (your choice) and likely not the latest and greatest software (because what developer thinks they'll make money off of you?). Perhaps you are okay with that. It's probably a cheaper route.
The cheapest option doesn't always provide the most value. The most expensive option doesn't always either. The question is, for the price you are going to pay, are you going to get the best value. That equation is different for everyone and no platform can possibly make it work for everyone either.
I own a SPARCstation 5. It's got a whopping 95MB of RAM in it and a cool 25MHz processor (I think). It runs lots of software just fine, but I don't expect it to run the latest FPS.
Maybe you own an iPad1. It's got a whopping 256MB of RAM. That's not even 3x the amount of RAM my SS5 had. You should be amazed it did all the stuff it did, and continues to do.
You shouldn't expect it to run the latest and greatest OS, though.