Ultimately the problem is Google. The minimum system requirements for Android keep going up with every release and Google stops providing updates to older releases at some point (typically 5 years after that version was initially released). That effectively puts an upper bound on the lifespan of any phone as at some point the phones CPU and memory aren't good enough to run the latest Android version at acceptable speeds. The lower end a phone was at original manufacturing the faster this all happens as well.

Apple is just as bad (far worse in some ways).

I've tried to find a solution, and the best I've seen is Linux phone, but that comes with some major downsides that are going to be deal breakers for most people. The two biggest ones are that battery life is abysmal unless you enable hibernation, but doing so, at least a year or so ago when I looked into it, disables your ability to receive calls while the phone is in hibernation. And secondly that NFC essentially doesn't work, or at least not for anything you care about like being able to make payments.