What story can you not complete now? Honest question. If there's some story related quest you can't do, a group can likely be arranged to help you through it. That's been done many times before.
But no, the game doesn't need solo mode "story" dungeons when it's already easy to get groups to complete things like that. With how limited the budget is, spending it on retuning things for another difficulty that generally isn't needed is going to take developer time away from something else that would benefit more players. The MSQ already has numerous catch up and helper mechanics like the Echo to ensure people don't get stuck.
I don't see how RP is a factor. I'm a roleplayer and it's not a barrier in dungeons. Occasionally with the flag up you even encounter other RPers in dungeons (although that's not terribly common).