The problem with Unreal Engine is (and always has been) that Epic makes the engine to make the game they're currently working on. So right now it is a Fortnite engine. Previously it was a Gears of War engine. (Maybe throw Paragon in between.) It started out as the engine for Unreal Tournament.

So if you want to take that engine and start making a different type of game, it's not necessarily going to have the tools you need. It's not necessarily even going to do what you need it to do at the base engine level. Not that it couldn't, but Epic doesn't give a shit. So they give you all the source code and support for building your own version of the engine so you can add the features you need.

You want to make a vast, persistent, open world with vast dungeons you can enter and explore? Yeah you're going to have to build support for that in the engine yourself. You want to do it without loading screens? Better get deep into that engine code. You want to have vehicles or mounts? NPCs, companions, AI enemies? When they hadn't added them to Fortnite yet, totally up to you to figure out, and probably through modifying the engine. Need to make major rendering improvements? Better dig in. Problems with the art pipeline lacking features you need

Every time you touch engine code, that's new tech debt. If a new version of the engine comes out, you have to integrate the changes. The longer the project goes on, the harder that becomes. Then Epic finally comes out with the feature you built yourself (say vehicles) but its only partly the way you did it. Now you're fucked and you have to decide right there: strip out your changes, switch to theirs and redo most of your work, or, stop taking engine upgrades and integrate new features piecemeal. Now you're in tech debt hell.

Almost every developer starts off with saying, "we'll use the engine as is, no engine changes allowed!" Three months later the cynical director is having a high level meeting about allowing a major feature get implemented in engine code. But it will be alright, they tell themselves. 3-5 years later they're up to their eyeballs in tech debt of engine changes, and realizing Amazing Game 2 either needs to be built using the old version of the engine they're stuck on from 2-4 years ago, or built from the ground up on a new version of the engine.

I'd be thinking long and hard before switching to UE5 if I were Bethesda. And they have the advantage of having access to some of the best Unreal Engine developers in the world (Obsidian, The Coalition) now that they're part of Microsoft. They're also probably getting a bunch of pressure to make the change as the studios create a corps of experts.

If I were them I would be very tempted to make the necessary changes to Creation Engine, and stay far, far away from Unreal. Sacrifice a year or two and your top engine devs to overhaul the pain points of Creation Engine, keep full control of your pipelines and versioning, and make the game you want to make, not the one Epic wants you to make. You can even make awesome DLC or a smaller sequel game on the old branched engine while the overhaul takes place, and just have a small core team working out the kinks on the new system.

I guess my point is, tech debt is not the point, because there will always be tech debt. It's a much bigger decision than that.