So I assume you bring this up in order to argue that Blizzard isn’t messing with the basic formula that made D2 great and people think that’s a good thing, so why is ok for Bethesda to mess with the formula of Fallout 2?
The answer, IMO, is that Blizzard has a legacy of know-how with the Diablo formula, the engine, what worked and what didn’t, etc. They can hit the ground running on development of a game that is the same as its predecessor, but updated to include some modern improvements to the genre and all the modern technology. There is a good chance they can make a game with the same qualities as D2 because they have the legacy of the work that was done on the original (of course, there’s also a good chance that they will move it too much into the WoW vein because WoW is incredibly successful and is something most of their staff probably worked on - the gameplay video is reassuring in the sense that they aren’t going to slow combat down to 1-2 guys at a time, at least, but we still know nothing about the skill and item systems).
Bethesda, on the other hand, doesn’t make games like Fallout, they make games like Oblivion. For them to make an iteration of Fallout, it would be very hard to make something satisfying to fans, and they would be charting waters they’ve never been in as developers. So it makes more sense for them to take a game type that they are good at and which has been well-received, and make their version of Fallout play more like that. Complaining about it doesn’t even make sense, since it isn’t like the Fallout development team is still around and Bethesda is preventing a sequel from them.