Its astounding what older devices are capable of with the right software. I installed the community supported version of ubuntu touch on a nexus 5 phone which is about 7 years old now and was stunned to see it was buttery smooth. This device was running faster than my pixel 2. Even web browsing and web youtube worked fine. Apart from the horrible camera and lack of an sdcard slot you could actually use this as a main phone quite easily.
except it has none of the apps you need if you belong to 99% of typical users.
People don't really need most installable apps. Just about every tool I use has a mobile website/PWA.
Some users might "need" Snapchat and Instagram (if we really stretch the meaning of "need"), but not 99% of them. A person who is willing to use old phones is probably more likely to not use those apps. I'm sure that many people wouldn't use them if it made their old phones work better. People who want those other kinds of entertainment apps can still spend money on new phones if they want.
You would probably find your days more productive and enjoyable if your phone didn't support the general garbage people install.
And since smart phones can run different apps, it’s not about the “one true phone”.
It’s like the old Slashdot feigned ignorance meme “Do people still watch TV? I haven’t owned a TV in 10 years”.
On monitor I choose what to watch, on TV I am forced to watch what someone else chosen.
I didn't own a TV ever (well, since I went to college), I see it only in my parents house when I visit.
Back then, they were also essentially the same technology (cathode ray tubes), but the TV had tuner and some analog circuitry to decode the signal.
The only way I see this could be done is if every oem committed to standard shapes for every part but that would never happen because they shape and layout of a phone is the only way they can make visual changes so people know its a new phone.
That's not the only logically sound story one can tell. Another story might be: porting OSes to old hardware raises their resale value, which means your new phones won't be competing against a cheaper version of itself. And people will be willing to pay more if they trust the phone they buy can be sold years later, or handed down their family.