I could care less about their engine. Skryim and Fallout were both visually acceptable and with mods are even better. They can continue updating the renderer forever, the core system works fine and is very flexible to do what you want with. They are the only one serving this kind of "open world, do what you want, cool npcs, cool lore, cool story, RPG, Crafting, Building" sandwiches on the planet. Every other rpg is a pos mmo or third person only, or overhead pos. Even diablo 3 was boring af and didn't evolve at all.
I think its Zenimax losing their marbles, BGS is working on starfield and a bit on ES6. Zenimax bought a bunch of studios to avoid being taxed so much, and these studios aren't making profits on the games they were known for so they let them make fallout 76, which they royally screwed up. I don't think BGS worked on Fallout 76 that much, if they had, they would NOT reuse 90% of the Fallout 4 assets they would make all new content. I think they are working mostly on Starfield.