I'm a little bit unclear about this, will Google's changes here also affect other android distributions like LineageOS, OxygenOS, etc? If not, then I could see that Google locking down their Android Distributions like this could breath a lot of life into some alternative distribution(s). If yes, then perhaps forks of Android or even competitors to android altogether.

Google has delayed releasing pixel 10 sources and unlocking bootloader for new phones is becoming increasingly rare. They may lock it down too going forward.

I don’t think so, but it’s getting harder to flash custom ROMs (locked bootloaders) and there are even legislations in planning which would make it illegal (at least in the eu).

It’s already cumbersome to run your banking app (and other „required“ apps) on a custom ROM with all the attestation going on. I assume these distributions will bleed users and see a reduction in new ones due to higher entry barriers.