Personally, I'm absolutely okay with a classical structure of "arcade" mode with individual character endings and simply waiting for the next game to see what sticks. BUT...
My personal preference doesn't matter, story mode is a huge part of what is making these games successful and allows NRS to continue to escalate the rest of the game with each successive entry. If it were canned, I would feel a lot less secure about the series's future and NRS's freedom to push the envelope in gameplay/presentation. So as far as I'm concerned, it's necessary for them to put in this single player content that the commercial playerbase loves.
FTR though, I think that their revamping of characters/mechanics and creating expansive variations of their movesets is a bigger limiter on the roster than the existence of story mode.