2) As you put it yourself, a problem with rogues isn't even the fact that there's no non-melee-DPS role, it's the fact that there are three melee DPS roles and not enough differentiation. Nuking one of the specs, and having to work two, distinct, melee specs and a ranged spec might be the way to go (I don't play a rogue to know if this example is actually good, it might be terrible, but for sure this change would sure interest me).
Will people bitch? Yes, but people are quitting the rogue class in droves right now regardless - only coming back if the class is OP which to be frank is shit design.
3) Same argument for your "slippery slope 4th spec" as I just gave above. People can deal with it, just like they dealt with 4 druid specs. People won't quit the game because other classes have more roles/specs, people will quit the game if their class sucks or doesn't have enough options rather.
Fact of the matter is, you can't look at slippery slopes and use that as an excuse to not change things that are clearly broken, or not implement things that would be awesome. If "others will complain" or "inch gives way to a mile" was used for everything that was done in WoW, we'd never fucking have nice things.
Just look at two things: Are things better before? Or will they be better after the proposed changes?