If things keep going in the direction they are, there might not be a LineageOS at some point, and developing a useful alternative before that (Linux based) would be great.

so we fork and continue to work on lineageOS. why start from scratch? (i mean, it's nice to have alternatives, but there is no reason not to continue developing an android fork.

Working on LineageOS doesn't help you if you can't even install it. Fewer and fewer phones come with unlockable bootloaders these days. The grip is tightening.

Yes, but if you are building your own phone hardware to run Linux on it, there is a huge advantage in that Linux flavor being an AOSP fork, since it is already mature.

While that's a viable option as something some people can do, it will be for the few in general, not the many.

right, but if you can't install a custom android, you can't install anything linux based or anything else for that matter. so that's a different issue entirely.