Hot tip for you, simply stop updating your iPhone and it won’t become obsolete and will last years longer.
Androids can be flashed with a new OS or reset to factory to give them new life.
It’s the same problem as desktop PC’s, Mac users stop updating their OS after a while and windows requires a reinstall from time to time. Even the tech illiterate often understand that.
Yes that has become pretty well known and puts people off buying 2nd hand, in reality it shouldn’t put anyone off as it’s not hard to keep one going for 5+ years.