Mostly, but there are certain conditions that must be met for Play services, for example. Users generally want this. They also want to use Internet Banking, and other services which may require a non-root retail ROM that will pass certain checks.

Some vendors like Huawei build their ROM from open source android (ASOP), or fork it and build their own to avoid dependency. Huawei is building HarmonyOS, which may not be compat with Android in the future. (Good luck with that Huawei....).

You cannot commit to this, it is not something that can be committed to. Security updates, for example, are mandatory. Without, your phone users get hacked. (And this can be 0 touch, remote, no notification, kinda hack.). Software is like a living thing. It grows and bloats over time. There is no didn't have bloat and still be useful to a majority of users.