I feel like Bethesda are being stubborn simply because they know the Creation Engine and getting something new would mean they’d have to learn all about the ins and outs, experiment, re-learn things, put money into it, cause longer dev times initially as they learn as they go etc.
They’ve said in the past aswell that the Creation Engine has certain elements and features that helps them pull off things within their open world games, things you might not get to do with other engines but I think if they re-learn a new engine they’d be able to get these things working eventually through trial and error. It’s not like the Creation Engine is a beast, look at how long it took to add climbable ladders.
I get them being reluctant, I do, it’s hard to let something go and learn new things but the longer they put it off the harder it will be later down the road when it comes to finally biting the bullet. If they haven’t done it for ESVI then I think they need to do it for Fallout 5.