That’s a years if not decade+ long project though

Yep.

You can’t just stomp a new game engine out of the ground

I don't know what you mean by that, but creating new game engines and migrating from one to another have both been done before.

Is either of those tasks fast or cheap? Of course not.
Are they worthwhile? Sometimes.
Are they possible? Absolutely.

especially not [...] if you want it to be as moddable as their current one.

Well, I can understand why you might assume that if you don't have a lot of experience in software development, but it's just not true. Making an engine that allows for very moddable games is mainly about planning for it during the design, and either building good tools for the game data or publishing the specs so other people can. It's not arcane magic.

(And for what it's worth, while Creation Engine is quite moddable, it has enormous room for improvement in that area. Actually working with it can be a very frustrating experience.)