Starfield is basically a game that's impossible to have an unbiased discussion on. Just by criticizing it you paint a target on your back, and same for when I praise it, though it does have a lot of flaws. I think for the Creation engine in particular it's not only about dodging royalties from using another engine, it's about what they've already put into that engine, and how comfortable the team already is working on it, and the proprietary parts of it that allow for the modding community, console command knowledge, and radiant systems to come along into new Bethesda games.
I would be quite interested to see them attempt working with a new engine and getting over the speed bump of adding these specific systems and implementations into a new engine that works better to begin with, but only time will tell when they finally find that worth it.