Phones have been getting better every year, and with it, new features and utilities enabled, throttling or not. No one (few outside HN) wants an old phone. People want the new features that are enabled by new hardware. The effective life of a phone is not limited by a battery but the remaining hardware.
Lot of iPhone users seem mention longevity as main feature. I personally know both Android and iPhone users, who like to have phone longer than 2 years. Having user changeable battery doesn't take away your freedom to change phone more often, but now you have phone with aftermarket value.
Phones are good enough that they feature sets last 6 years, while the batteries don’t. Plenty of people don’t want to drop $800 on a new iPhone every few years when their old one works fine. And those who do move their old iPhone to the secondary market. The secondary market exists because contrary to your statement, plenty of people are fine with old phones.