The thing is, AOSP is open source. Just like Linux is not controlled by one of the TooBigTechs, I could imagine a fork of AOSP becoming a viable alternative without depending on Google anymore.
And supporting alternative Android OSes (like GrapheneOS and LineageOS) is one way to support that vision.
Linux on Mobile is nice, but unfortunately I think it's an uphill battle because of all the proprietary firmwares?
In my GNU/Linux phone, all drivers are FLOSS and all pieces of firmware are backed into the replaceable peripherals (modem M.2 card and WiFi M.2 card).