For as long as I can remember, it has been an absolute mad house leaving MK after the fireworks. I think it's a bit naive to expect any different. People are already pushing their kids too late into the evening to watch the fireworks because they think it is a "must do" for their Disney experience despite exhaustion, and for some, it may in fact be a "must do." For the others, even if they left the park open, I think the vast majority would still leave after the fireworks because it's a natural end to the day. I don't like dealing with the crowds, and so I have to make a choice of whether I leave early, or stay and shop after the fireworks. I also make the choice to stay at WL so I can just hop on the boat, which never seems to have a crowd issue. So, you do have some choices to avoid this.
I have to assume Disney is already running max monorails at that point. There is no point in widening the sidewalk to the monorail if you can't fit the people on the monorail. I can see multiple reasons for restricting the population getting on the resorts monorail. Not only is it a traffic flow issue, but also a security issue. I suspect they don't want to induce people to hang out at the resorts who are not resort guests at late hours. It's better from a liability perspective to force those people to the TTC and away from the resorts. Obviously, they cannot prevent other means of getting to the resorts, but this certaintly decreases the likelihood.