Technically there is UnrealCLR, so if you made a game with mostly Unity-agnostics code it's possible to keep a bunch of it.
One of my game is multiplayer, the whole gameplay code is completely Unity-agnostics to run server-side without any Unity code.
Which is a proof of concept and doesn't do mobiles, game consoles and VR/AR headsets, mostly useless.
Very true, for now, but the amount of tools / libraries / projects that will spawn / funded / contributions to move projects out of Unity is gonna make that more realistic eventually.
I seriously doubt it, as it requires embracing the game development culture that most FOSS folks are against to, and most commercial engines rather stay with compiled languages instead of having a R&D department in compilers.
Or move to Godot/C#.
Doesn't do game consoles.
GODOT developers are trying to do it via https://w4games.com/. But not sure if it's ready