I like their engine,tho i do agree that it needs some updating and bug fixing. But it is one of the few engines that allow such massive freedom to the player.
I mean, most current engines support essentially everything Gamebryo does, but better, with fewer bugs and better performance.
Unreal Engine? The Outer Worlds has essentially the same gameplay as Fallout 3/4/ETC, with nothing missing. The worlds in the game are smaller, true, but this is a limitation of the game, not the engine. There are
several open world games created on it.
Unity is a bit of a mess, but certainly no worse than Bethesda's. There are also several games with a gameplay that's very similar to the TES series, with open world, interactive scenarios; like Rust, Outer Wilds and Subnautica.
Cryengine is a bit harder to work with, but it also has a
good list of open world first-person games, and is infinitely more stable than Gamebryo.
There is almost nothing that Bethesda's engine does that other engines don't do better, more efficiently and with greater stability. The whole point is that they would need to carry all the code and all the mechanics essentially from scratch to the new engine, which would be a huge job. But it wouldn't be more work than trying to make their engine even remotely functional, I bet. But no, the fact that they don't do this (besides the obvious reason of not wanting to depend on an external engine) is the structure that already exists that makes Gamebryo super modder-friendly. Bethesda isn't (that) dumb, they know that, by far, one of the factors that most influence the longevity of their games is how easy the internet can produce mods in a matter of days. In a year, the game already becomes something completely different from the original release.
Other engines were not built or adapted from the start to be so modular. That way, they would have to either invest a ton of time to try to make the engine more accessible and easier to create mods (which is an extra job and can still make the engine more unstable in the end), or they would have to abandon the idea of having games that are simple to create mods to, and since they've already cultivated a gigantic community used to creating content for their games, that would be suicide. So no, Bethesda won't change engines anytime soon. If ever. And in fact, they're to some extent "stuck" with some of the engine's problems because they're trying to keep it accessible and "basically the same" in everything that concerns the mod creation process. Which obviously doesn't justify that they still have stupid bugs from the Oblivion era that haven't been fixed yet, even though they've been fixed by the community in basically every new game they release.
It seems they're not even trying.