The issue with batteries on older iPhones isn't even replacing the battery. Apple will do it for like $80 bucks or so out of warranty. That's WAY cheaper than a new phone.

But every new OS version manages to use more CPU and GPU and burn down that battery faster even if it's brand new, since the older chips have to work harder to run them than they had to work to run the older OSes.

I replaced my battery which was showing around 83% of original capacity last year, in a 3-4 year old phone. I was skeptical of the 83% reported number. Nope. The new battery didn't last much longer, nowhere close to how long it lasted on the OS it shipped with.

(This software-cpu-bloat is not unique to Apple. My Pixel, after 4 years or so, was practically unusable just from the amount of background shit the CPU was doing, compared to when it was new.)