Bethesda makes games other than RPGs for one, which tells me they know how to make more than one sort of game.
Secondly, their work on Oblivion shows they are are capable of addressing flaws in previous titles.
The merits of a Bethseda made Fallout are many. We'll likely get good mod tools, community suggestions will be implemented, we'll get a well polished game, and there will be plenty of support after the game ships.
I'm not saying there aren't flaws, or that I wouldn't have prefered Van Buren. But I'm not writing off Bethesda's FO3.