Hmm I think my main concern would be lack of kernel/firmware updates, running something like postmarketOS could partly solve that and still be nearly as easy to set up (just unlock and flash a prebuilt image)
But firmware is still almost entirely dependent on the vendor, since it's all signed and unpatchable.
Next issue would be lack of connectivity on a lot of phones, which have gone backwards and include USB 2.0 now. WiFi is an option, but less stable, I personally decided to just go 100Mbps and suffer.
As for the battery, it would help a lot if phones were designed to boot without one and they were removable, it all worked well for about half a year until I found out I had a spicy pillow and had to replace it with direct power to the board, which made the whole setup much less elegant and required soldering.
It all comes down to how devices are designed in the end. If someone took the time to make a computer instead of just a phone, and included features that make it useful past its initial life that aren't that popular (display output, microsd, headphone jack), mainlined all the drivers and maintained firmware, that would be a different story.
But that's not a very profitable model, because it's all about reducing waste and thus selling less. A lot needs to change.
Won't work sadly, if you install a custom OS your device will not be able to attest to it being original, and play integrity won't pass (which would by extension include WEI). Not providing the results will be seen as just as bad as not passing. So as long as the vast majority of mobile users have it deployed you're screwed.
You can think of it as requiring everyone to wear a cryptographic ID badge to do something as simple as going to the store to buy groceries. You can always not wear it, but you will be denied service just as someone who has a "made up" ID.
The evil exists at the silicon level where they cryptographic keys are hidden from the user.