From a design standpoint, there's a lot of reasons why they wouldn't do this. Firstly, it cheapens the individual significance of each of the personalities of the Warlords. The entire point (lorewise) of doing an expansion like this is to utilize personalities in a way Blizzard never thought they'd get to. There's no way when they were writing Warcraft 2 they thought "Hey, maybe we shouldn't kill these guys in case we make an MMO and want a chance to explore their characters on a much more personal level." Killing the Warlords in a "council" style fight feels cheap.
You then have the actual mechanics of the fight. If you have them "rotate" out, the fight feels cheap. You're talking about 20 man raids for progression now, which means they can't really design having more than 4 tanks without things getting really weird for the fight. While you could do some kind of cool DPS-offtank mechanic, this just feels mechanically cheap. Either the encounter has to give the players the tools (deus ex machina) or it has to be so absurdly specifically designed that it essentially is going to require an exact 20 man running an exact spec across the players in that raid that it just becomes a "shove the triangle into the triangle hole" type of thing.
So it's limited either in impact or mechanics, and neither makes for a good expansion conclusion.
Secondly, what's the point then? Sure, okay, we kill the Warlords, but where does WoW go from there? Every expansion has ended with "Bee tee dubz, this is coming your way." BC pointed at the Lich King (sort of), Lich King pointed at Cataclysm, Cataclysm sort-of pointed at Garrosh (if not Pandaria itself), and now Pandaria pointed at...well, Garrosh again. It left room.
There's no room left if the xpac ends with the Warlords. We become wholly responders to an event, there's no initiative taken. BC was because we chased Illidan, Wrath was a response-scenario (The Scourge's War, albeit it was a war we saw coming). Cataclysm was a pure response event (whoops). Pandaria was a really organic movement of conflict through a new setting and in that regard particularly was well-written.
If the goal is for players to feel like a protagonist, it can't be a constant "response" scenario and that's what ending WoD with the Warlords does. We need to take initiative in some way. We fall into Superhero Syndrome otherwise, and if you didn't know, Superman doesn't do jack-diddly-squat until Lex Luthor starts stirring things up