Bethesda didn´t change their standard post- daggerfall combat because of Diablo, or Diablo 2 or Mass Effect, or whatever other successful action rpg release.
Why would they do it because of KoA? Most fan´s dont play rpg´s because of the flashy combat, they do it because of the sense of world building and immersion, else Planescape, baldurs gate, the witcher or the elder scrolls games would have never had any kind of critical or fan acclaim, as their combat systems were usually counter intuitive.
I'm not praising Amalur because its combat is "flashy." If anything, the only real difference in combat from Oblivion to Skyrim was that it became more "flashy," what with dual-wielding spells and the like.
I am praising Amalur because its combat is a good gameplay mechanic, unlike Skyrim. I'll agree with you and say that the world of Skyrim is probably unparalleled in video games currently in terms of depth and breadth, but at the unfortunate sacrifice of gameplay. It is a blast to spend hours on hours discovering new places, people, stories. But it is not fun to fight.
Why would they change their combat after Amalur, and not Diablo or Mass Effect? Simple. Diablo is a top-down "clickfest" action RPG, whereas Elder Scrolls games have always been 1st person oriented (with the option of third person, but who plays that way?). That's like saying Elder Scrolls combat didn't change after Final Fantasy 13 came out. Why would it? They're absolutely nothing alike! Mass Effect has gone on to become a third person shooter/RPG hybrid, which for obvious reasons has little bearing on the first-person oriented action of Skyrim. I don't think anybody here would argue that Skryim and Diablo are the same "type" of RPG. Same goes for Skyrim and both the Mass Effect and Dragon Age series.
Amalur, on the other hand, although a third-person game, is comparable gameplay-wise with Skyrim in a lot of ways, more so than any other western RPG out there right now. Almost everything outside of combat is highly comparable between the two games. While I'd give Skyrim the nod in 9/10 categories, Amalur's combat is light years better than Skyrim's. Because Amalur has situated itself as such a close competitor to Skyrim (whether or not you think it is as good of a game is irrelevant, they're still selling nearly identical products), Bethesda would be foolish not to pay attention to what aspects of the genre BHG has clearly done better than they have.
In summary, I think Amalur has carved a niche for itself in the increasingly crowded WRPG market by creating a fun and unique (for this genre at least) combat mechanic. Mass Effect 2 greatly improved on the ME series' combat mechanic and ME3 looks to be the definitive version. As I said earlier, DA2's combat was great (even if the encounters weren't) and different than everything else. Demons'/Dark Souls combat is brilliant and makes the entire game, and is different than anything else. In my book, that leaves the Witcher 2 (combat was very "meh," worse than Witcher 1 IMO) and Skyrim floating without clear direction about where to take their combat.
People want to play games with fun and riveting stories, and be immersed in them, sure. But I don't think we have to settle for shoddy gameplay, either. As I explained above, Skryim is really behind the wagon in terms of combat when compared to all other WRPGs... If Bethesda wants its next Elder Scrolls game (or Fallout game!) to really be the premier WRPG, it needs to learn some lessons from Amalur.
