I also still have my four-year old 6, and it’s working just fine. I took advantage of the battery replacement offer last year as well. When support for apps goes away, I’ll upgrade. There are other things I’d rather spend (or save) my money on (or for).My 6 (with a recently replaced battery for $29) is perfectly fine. I may switch phones soon, but only if I switch to Google Fi for my carrier, in which case I'll buy a Pixel.
You can replace the battery for $29.I have the SE and my battery barely lasts a few hours now so looking at switching to the Xs soon. Very soon!