you made a poor decision on phone
Not really, it's an S4 and it has served me well and I've been sim only for ages. If I had stayed with operator updates which is what I am referencing then I would not have had an update for years.
Why should I have to buy a phone every two years?
Personally I'm on Android 6 (CM 13) and as soon as lineage goes stable I'll review and maybe install that but then I'm different to the average user. If lineage isn't any good I'll start rolling my own based on CM 13 with the updates applied.
Once you have a phone that does everything you want it to do then there really isn't any point in upgrading. I just buy a new battery every couple of years.