Which kinds of issues, though? Bad animation, graphics that manage to look outdated and washed out?
Yeah, that’s fair, it’s unclear how much that’s Bethesda and how much the engine (though I am fairly confident that the animation issues are engine-linked at the very least).
Other stuff though, like the hideous and gigantic amounts of effort required to make NPCs do, well, just about anything (and this amount is increasing as time goes on), the terrible pathfinding, the 2.5D terrain, the hard-linking to framerate for physics/event calculation, and so on, and so forth that’s definitely GameByro-specific stuff.
And it’s not GameByro’s fault, really. The NetImmerse/GameByro engine as we know it was designed for client-server model MMOs, and had features very much specifically designed for that environment and era (the first game to use GameBryo, AFAICT, is Dark Age of Camelot, and it was fucking amazing back in 2001). 2.5D terrain and simple NPCs and pathfinding and so on was normal back then. Physics was a weird thing a few mods and games were kinda messing with, not a normal thing in MMOs.
Bethesda presumably picked GameByro for Morrowind because their game was quite similar to an MMO in terms of requirements, back in 2002 (and presumably earlier), with a big 2.5 world, which you could stick caves and stuff in as instances (forgive my language here, you know what I mean), the ability to have thousands of NPCs with different stats who existed in this world and could potentially go about routines without the player around and so on (although that’s kind of a big trick and relies on the scripters being smart about it), and where it was fine, even good for the NPCs and players to have essentially the same kind of stats (i.e. not highly asymmetrical).
So it all makes sense.
The problem is that games moved on, and Bethesda has tried to make the engine move on, but has only had limited success in doing so, because they basically have to make fairly hack-y changes to it, ones that work better on the graphics than the gameplay. This is why physics is such a goddamn hilarious disaster in Bethesda games, because they had to hack it in to an engine not really designed for it.
They are also, IIRC, using an older version of the GameByro, and they definitely licensed it or bought in such a way they are no longer bound to GameByro (which kind of seems like it stopped development a while back).
So a custom-designed engine or a just a genuinely modern engine would probably be a pretty big upgrade. If they still had janky animations, well, yeah, then we can “OH GOD, they’re just talentless!”. It does seem like they’d be looking for an engine perhaps more akin to a very modern MMO engine than a shooter engine, say, given the need for NPCs to be able to “do their own thing” (even though it’s mostly an illusion), which is kind of a core part of “Bethesda games” at this point.