Don't worry, it WILL bomb (as in much less than what they were expecting). Lots of reasons why, the main one being that the first was divisive enough, which means that not everyone that bought the first will get it again.
It'll sell good enough though.
And considering the game has 0 marketing and hype besides the fanboys and internet dwellers...
Yeah, I don't see it selling over a million copies (PS3/360 combined) like the last game. I predict 500 000 copies total. Like you said, not everyone who bought the first game will buy this, and those who didn't get LoS1 will probably not jump in for the sequel. Well, some will, but I don't see many people doing that. This is, after all, not GTA or Assassin's Creed. Sequels generally don't perform as well as the first game.
On top of that, one might wonder if releasing Mirror of Fate on consoles is such a great idea. Modern gamers, unfamiliar with the LoS saga might try it and decide that LoS is not for them (even if they would potentially like LoS2 much better. They would write the series off as a rough-looking 2D platformer and not give LoS2 a chance.)
I am personally more interested in LoS2 than either of the previous two games, but I don't see it sell a lot. I don't see reviews being great either. Possibly in the 6.5-7.5 range at best.
Though I know absolutely nothing about this. Just speculating for lack of better things to do.