"Many people", let's say a total of 40,000 would pick up the game for any or all the characters you mentioned, they don't care about mechanics or anything, just their character and the story mode. Now compare to the 15,000 on the fence because either they didn't like MK or don't like the way this looks but really like fighting games. Do you try to reel in those 40,000 that will play their favorite characters for a week or two, play the story mode and then trade it in (some will stick with it, many won't) or do you you try to reel in those 15,000 that will keep the game for months on end (many will, some won't) and possibly buy the DLC as they like the mechanics of the game?
Completely out-of-my-ass numbers but this scenario can easily play itself out and has with other genres before. If one character is truly all you need to purchase this game then you don't care about the game as a whole more than likely. Whereas the people that want to know about the mechanics care about the game beyond the characters and will continue to support it.