I'm continually mystified as to why companies don't want to release the old technical documentation and software. Is it all so bad that they are THAT embarrassed to show it?
The changes for the company in releasing old software is minor, the vast majority of users don't have the skill to deploy it and people who do have the skill can earn enough money doing a variety of technical work that repairing old phones isn't going to be an attractive option.
What portion of phones capable of running LineageOS etc end up being used in that way? 1%?
Reprogramming the 1000 other devices won't be as hard as the first one but it won't be trivial as they may be all on different versions of the software and there may be hardware variations too.
Just to triage the devices and determine which ones are good enough is going to be non trivial.