The thing is though, there is no open source project even close to what Unreal Engine currently offers. It's just lightyears ahead in everything kinda? Editor tooling, asset pipelines, advanced rendering features, ... While you could build on open source tech and implement all the missing pieces yourself, that is just not going to be a sensible investment for most studios.

You'd be hard pressed to compete with Epic Games' investment in the engine for their own games and others, too. I don't think this situation is comparable with Hashicorp. Epic has more than a hundred engineers continuously improving the engine, doing research on new technologies like Nanite and similar, and all their improvements are available to you immediately. You have full source code available and are able to modify it as needed, you can also fork it under the current terms. As far as I know there were never any forks with significant traction, only to implement random features people needed in their game.

This is just a guess, but I truly don't believe we are going to see Epic Games turn evil like Unity has as long as Tim Sweeney holds more than 50% of the shares. Going public was Unity's death sentence - the tech no longer mattered, now it was all about extracting the maximum value possible before the company is run into the ground.