ID tech is nowhere near flexible enough for something like Starfield or even Skyrim. It's partially the reason why it's so efficient. It simply isn't fit for the task.
And the Bethesda developers are intimately familiar with Creation Engine, achieving the same level of productivity with something new will take a long time. Switching the engine is not an easy thing.
Not to say that Creation Engine isn't a cumbersome mess. It has pretty awful performance, stability and is full of bugs, but on the other hand it's extremely flexible which has allowed its games to have massive mod communities.
If Bethesda can't take the time to do it then who can? People act like they're some small time developer but they're not. They simply refuse to expand their dev team to do things like a redesign.
Creation engine is not going to hold up well for another 6 years, there's no way their cell loading system will be considered acceptable by the time ES6 comes out. The amount of loading screens in Starfield is insane for a modern game. This company needs new talent badly.
I know they don't want to switch, but it would be worth it to make the swap to something like unreal, even if it takes a few years of customization to get the open world stuff right. Creation Engine just feels so old.