I have no issues with my 4S. I did in my 3rd year with my 3G. Early in that year, a bunch of my pass wouldn't run anymore, when iOS 4.3 came out it was the las available OS upgrade for the 3G. It allowed all those dead apps to run again but at the cost of a drastically slower phone. I couldn't wait to upgrade to the 4S when it came out at the end of my 3-year contract. My 4S contract expired last fall, but I haven't bothered to upgrade yet: all is working fine. I'll now wait until the next phone comes out.
I think Apple is working very hard to keep forced obsolescence to a minimum. OS X updates used to really only be available for macs for about 5 years after they were built, but my old 2008 iMac is running Yosemite with no problems. In fact, since the latest Yosemite upgrade, my old computer seems to be running better than it has in a couple of years.
I think the problem with the early iPads and iPhones is that they were such a new technology, that subsequent models had a lot more room for improvement. New models today are evolutionary, not revolutionary. Whereas the first few models took huge leaps over each other with each new iteration. It would be like asking a mid-80's Mac to run OSX.