since all apps are designed to run well on budget phones from 5 years ago, there's no reason to upgrade.
5 years, maybe, but any more is stretching it. And not getting system upgrades anymore is problematic. Unless you own a particular model of phone, de-Googled Android can be hard to come by.
For example, I have a 7-year old Pixel C. By the time Google stopped using system updates for it, I wasn't wanting them as every release made the device slower and more unstable. After some effort, I was finally able to install a version of Lineage, which itself has problems including no updates in years. There's a lot of software that is incompatible with my device, both from Aurora and FDroid.
Android isn't Linux; Google doesn't care about maintaining backward compatability on old devices, much less performance, and there's no army of engineers making sure it is because there's a served running in walled-up closet no one can find.
Google deprecates features and ABIs in Android, apps update and suddenly aren't backwards compatible.
5 years, maybe. The entire industry is addicted to users upgrading their phones, and everyone gets a piece of that pie. There's no actors, except perhaps app developers, who have any interest in keeping old phones running. Telecoms upgrade their wireless network - the internet connection in my 8 y/o car, and half its navigation features, died the day AT&T decided to stop supporting 3G; Phone makers make no money if you don't buy new phones; and maintaining backwards compatibility costs Google money which they'd rather siphon off to shareholders.
I'm 100% with you. I want a Light Phone with a changeable battery and the ability to run 4 non-standard phone apps that I need to have mobile: OSMAnd, Home Assistant, Gadget Bridge, and Jami. Assuming it has a phone, calculator, calendar, notes, and address book - the bare-bones phone functions - everything else I use on my phone is literally something I can do probably more easily on my laptop, and is nothing I need to be able to do while out and about. If it did that, I would probably never upgrade; my upgrade cycle is on the order of every 4 years or so as is, but if you took off all of the other crap, I'd use my phone less and upgrade less often.
The main issue with phones like the Light Phone is that there are those apps that need to be mobile, and they often aren't available there.