I don't think you'd ever want an engine level monopoly to that degree, even Unreal isn't by itself capable of the systems that allow Starfield to work the way it does, and would require serious modifications to do so, and not every studio would perform those modifications necessary to complete their game's vision, and then just give all of that to everyone else to piggyback off of for free, there are a lot of reasons to not do that, specifically, what Unity is doing now.
It only seems cool to do that with Unreal because they haven't pulled anything like Unity... yet. Not having done that yet doesn't preclude them from doing it, that's the scary thing about the Unity debacle, anyone engine could turn around and make a horrible change, we just have to trust that they won't, and being given monopoly power makes it too tempting to trust forever.