MSFT could have opened up idTech completely since they make 0 dollars from licensing the engine anyways.

Microsoft's game divisions make money through making games, so opening up the engine itself would've been conducive to their goals (cultivating an ecosystem of devs and even contractors familiar with the tooling).

idTech rendering is more competitive with Epic's Unreal technology than Unity and Godot.

They should simply open source it if they fire the devs. Else the engine and future support for the games built on it are essentially being tossed into the trash.