I think the big question is whether they “improve” the APIs these apps require at the same time.

That would break the alternative implementations.

Android APIs are stable and an app will keep working until Google decides to bump the min SDK version in Android itself. The APIs a Google app uses are the same as any other app. Unless the app is privileged ( = can use system APIs ) there's no difference in capabilities.

Actually there are a few APIs you can't use when the app isn't signed by a system key.

All oems or rom makers are able to make whatever apps they want system apps. So even if this is being done, Google dialer isn't getting access to something others cannot.

Yes. That is the problem I am referring to. They can bump the api version, removing the non-privileged APIs the AOSP stuff used, and move the implementation of the new privileged API into a proprietary blob.

This is how location services work. Location has an obvious local-only fallback, but it is broken in practice. I imagine they can break phone calls/sms/rcs equally badly using the same technique.

Oh wow, you're one of the guys from LineageOS right?

There's many of us :)