Not worth it for phones right now since they have to use proprietary hardware to connect to LTE/5G networks and the older networks are available in less and less of the world.

You'd do all the work of replacing all the system components you can, only to be left with a totally incomplete job due to that widespread LTE/5G adoption.

Better to focus on what's needed at a given time. Like, with "sideloading" (choosing your own apps) being banned from Google-certified versions of Android, we really need all our phone manufacturers to make non-Google-certified versions of all the Android phones, even if they still use Android.