an ancient game engine that’s probably barely hanging together

I think Bethesda is a company full of people at the terminus of their careers - they don't know how to make any other kind of game than "Bethesda RPG," they don't know how to use any other game engine, and they are unable to learn either of those skills. Many other game studios have learned to evolve and shift their resources and assets - Naughty Dog doesn't still use the Jak and Daxter engine, From Software went from making mecha games like Armored Core to defining an entire genre with Dark Souls. It seems like Bethesda doesn't have the capacity to change like other companies.