I can't really speak to traffic because I rarely drive, but I do agree there needs to be more investment in rapid transit. I rely on the red line/walking to get to and from my office. It is the only line I would consider living on.
But I did want to add that it isn't only the north end that was cut off. I doubt the resurgence of the Sea port area/"innovation district" would have happened if it hadn't been for the big dig.