They've made their intentions clear. As soon as third-parties start to use adb for sideloading there's a very good chance they start to lock that down as well.

That is speculation. Is there any evidence?

None that will satisfy your standards, given I think only the announcement from google themselves will.

Was there ever evidence that they would take away apk installing?

The current trajectory provides at least strong evidence.

If adb installing is used to circumvent their signing programm, it has to go as well.

adb backup is gone