My 2011 Macbook Pro is still perfectly usable for everything a normal person would want to do with a computer. So is my parents' 2012(?) Mac Mini. I just bumped the RAM up to 8GB on both of those computers and they are just fine for web browsing, word processing, and HD video playback.
I suspect that Apple's problem selling iPads is related to this -- my iPad Air 2 is never noticeably slow doing any task, and I know people with pre-Air iPads who are perfectly happy with them. These are people who have plenty of money to buy a new iPad and would do so if theirs felt slow; they just don't notice any drawbacks to using an older one.