Nobody complained about having to re-roll as a DK, Monk, or DH because none of them ever existed before and if you wanted to play them, you had to make a new character. If they do what you're suggesting, anyone who has a hunter already will be able to play the new spec on Day 1 while everyone else has to boost or grind. Taking something as pivotal to plot as DKs to the LK, Monks to the Sha, or DHs to Illidan/the Legion and reducing it down to 1 of 4 (all DPS) specs on an existing class would be stupid. Especially when they've already shown (per DHs) that they're fine to have a class with only 2 specs so long as they are different enough from each other.
The many changes to hunters over the years (and also rogues for the same reason) is precisely because it's really hard to create 3 distinct dps 'feels' alike enough to be the same class but different enough to feel make players care enough to choose between them (rather than just playing whichever sims the highest). Also, you gotta go back to Vanilla to understand why Survival got the rework that it did. Back when hunters had to carry around ammo and 2 weapons, they had a melee/range hybrid spec called Survival. At some point Survival was adapted to be a PvP trap-focused spec, but traps were a pain in the ass so they added a trap launcher mechanic, and then I guess finally decided to roll that back into MM and go back to a Vanilla-esq Survival (for the exact reason I mentioned at the start of this paragraph).