> I guess because from a user point of view it doesn't make much sense: by the time the battery is dead, the phone is so outdated it serves no use anymore
My previous phone lasted four years, and really I only replaced it because I have more money than sense and wanted a 120hz screen. iPhone 11 Pro, came out nearly five years ago, still totally fine, lots of them in circulation. The battery was certainly beginning to show its age by the end, but the phone was otherwise fine.
Your point was maybe valid a decade ago, but phones last much, much longer these days. And getting supported still longer; current phones, particularly on the high end, are far faster than they need to be to support the OS, and rate of hardware change has slowed significantly (Apple, say, hasn't had a full-blown microarch change in years, with the M1-4 and their sibling A-whatevers being very much incremental progress on each other).