Arm-chair babbling idiot who plays too much video games here, I am one hundred percent convinced that it has nothing to do with visual fidelity and everything to do with that asthmatic engine they've been dragging since Morrowind.
Code doesn't go bad with time, that's not really how it works. And game engines tend to be a Ship of Theseus situation, where just because it's still the same "engine" in theory, doesn't mean that large parts (or all of it), haven't eventually been replaced or refactored over the years.
Unreal Engine has been around for 30 years at this point, would you also consider that an "asthmatic engine"?