Well Unity Made a announcement to make Devs pay per Download and many devs straight up said their games will be deleted the day these changes are made.
Reddit didn't retroactively try to steal money from developers. Also a game engine doesn't need a community to exist, it just needs to be good, a community is helpful but not required.
I doubt that, devs can switch Code, Shure some game devs need to remake already written code but i think there will be someone making a code translator right now.
Even if you could just "translate" code from one language to another, that ignores asset pipelines, asset store libraries, and all the build pipelines that allow you to ship cross-platform.
You also need to now train your entire dev team on a new tech stack.
If you can do it with databases you can do it with most other code. Shure it won't be problem free but way better than bankruptcy. And users will understand that it might be buggy for some time if you explain it to them.
And yes you have to retrain your staff but its their job.
And of course there will be library issues but there will be someone making new libraries.
I'm using Godot 3 for my current project because even the relatively minor changes I'd have to make to port it to Godot 4 would be unfeasible. If I had to change engines entirely I'd have to just abandon the project.
One of the biggest appeals of modern gane engines is that you barely need any code but that also means everything is centered entirely around the game engine, I doubt there is any way to transition that, it probaly means devs have to start from scratch and reimplement the mechanics.
I've hated Unity since its buggy trash first showed up in flash games Sure they ironed out the bugs and it went mainstream, but I never forgot how it shouldered it's way into the picture. Now it's pulling this shit and I've got that inevitable mixture of smug and disgusted that accompanies the all-to-familiar experience of "I said this was a bad idea but did anybody listen to me? Nope."