It's funny you say that, because the one thing I think the TES games do badly is the combat.
Arena was originally supposed to be just that, an arena, and the combat was designed that. It was an arena combat game that grew into an open world RPG. But the sloppy action combat remained, and it still does.
In recent games, TES has actually started to streamline some other features to its detriment (like Skyrim's stats and abilities), while at the same time fixing some of the fundamental problems of the eariler games (Oblivion's broken level mechanics), but it sticks with the action combat. Worse, BethSoft has recently demonstrated that they can do combat better than that, with the VATS in FO3, but they still don't improve it in TES.
I have no idea how VATS works (whenever FO:NV gets on sale I probably will find out), but TES is heavily based on melee combat. You've got your archery and your spells/staffs, but the overwhelming majority of the enemies you encounter are going to try to eat your face. When was the last time that a first-person game, that was also melee based, was true turn combat?
I'm no authority, but outside of VATS (which again I have yet to encounter), I don't even know of any recent first person game, disregarding melee, that has turn combat at all. It's something you just don't see anymore.
Edit: I feel this is the place to snark-ily point out what I think they do wrong, the roleplaying
But it did improve the combat. Vastly. I used VATS for nearly every attack in FO3 and NV. It was as vital to me as the pause-to-aim feature in ME.
I feel obligated to again point out that for us snipers, pause-to-aim in ME was useless.
I suspect big budget games are vanishing because their iteration rate is very low. Indie games come along and they don't have as high a quality but they can iterate relatively fast until they find something that works. Then larger companies form around this and can milk it for a time.
But I think anyone who thinks they've hit on a kind of game mechanic that will always be popular unaltered is kidding themselves. And when you realise that the emphasis for long term survival switches to, 'How much can you test against the market?' not just 'How much money can you sink into producing the brightest assets?'
Then again I may be wrong. COD and Assassin's Creed are still around, for the time being 
I don't know. How often does an indie developer iterate? All the big indie hits I know of, they're ONE game. Those guys aren't pumping out 5 hour quirky games, they're making a single five-hour quirky game.
The thing indie has is volume, that's all.
Edit: And what does unaltered mean? Because Assassin's Creed iterates heavily.