It's VERY mod friendly, they have modded it to hell and back to fit their needs, they know it like the back of their hands (and yet they use it like shit), buyers don't seem to care. At this point changing it would cause a big delay for their next game, because they first would have to learn it, then they should modify it to suit their needs, then their first game using it would probably be more buggy and running worse than usual (!), the mod scene (one of the reasons their games sell so good on PC) would be slower to grow and mods that fix their shitty work would take longer to appear. They have nothing to gain from chaning it at this point. But the real issue isn't the engine itself. Plenty of engines are, at their core, super old. It's just that Bethesda doesn't have good programmers AND they don't design their games properly around the limitations of their engine. I wonder if the junk now being so valuable for modding isn't a design choice done so that the game won't have to remember as much pointless stuff as before.