The problem is how locked-down most phones are, and how hard it is to modify their software. Even for the Fairphone, you have to fill out a form on their site to get a bootloader unlock code, and they could close that form if they wished (see Asus). That all means starting an "ecosystem" of accessories and new non-phone software is costly and has an uncertain future.
Personally I think the biggest issue is the theft-prevention functionality that means a phone picked out of e-waste is basically bricked (without some exploit). There's companies making new motherboards out of salvaged Intel chipsets, I'm sure it would be possible to build a business around the reuse of phones, but right now there are just too many obstacles.
I think this could be solved with new legislation. At least here, doing anything with e-waste is already highly regulated. Giving registered e-waste processors the ability to unlock the bootloader of any device would reduce waste, and make unlocked phones something you could reliably buy in bulk. Then I think we could see the kind of aftermarket support for phones.
You also need to sign up with Google to even get past the setup screen, and the phone needs to reach Google's servers and ask for permission to be used. Even if Fairphone would like to keep phones usable, Google can decide otherwise at any time.
I started a thread on this topic on their forums, and they seem to have no interest in fixing this. I wouldn't consider hardware sustainable if it needs to talk to Google's servers to be used and remains completely locked down otherwise. If you find one of these devices in a drawer in 15 years, and Google has changed their server's API, then the phone is as usable as any other brand
(nitpick: you have to "enter a contractual agreement" with Google, and not create an account. Folks on the forums seemed to be obsessed with the choice of word around this, although practically, it makes no difference).
If you go with the Googled-Fairphone, then it is Googled indeed.
I bought a Fairphone 3 a few years ago with /e/OS, so I don't have that problem. Also in all fairness, the "you have to log into Google before booting" is making it harder to steal and resell phones. I read somewhere that the number of stolen phones got lower since they (Google, Apple, I guess Samsung and the likes) introduced that "protection".