And exactly where are you going to put all those people during parades and Wishes????? It's already wall-to-wall people at those times.
I'm not sure where you are getting that from my post, as I specifically said that MK should get additions -- as long as the other parks are getting additions at the same time, particularly ones that are more attention grabbing. That way, anything coming to MK would be more "under the radar" and less likely to increase crowds there while the other parks will hopefully increase their attendance and spread guests around more. The way I envision it, they do something like open Star Wars at DHS in 2019 and advertise the heck out of it and be like "oh, and there's also this addition to MK you can check out" as sort of a low key thing.
Ironically, I think that if they actually built a high capacity ride based on WRE in Frontierland that it would be perfect for that purpose. It would likely be low key enough (albeit hyped in fan service) to not drawn particular interest among your average guest and drive attendance, while it could improve the number of attractions at the park. And if it were a slow moving and longish ride, could even help keep some folks off of guest areas and inside a show building.
That said, more to your point, the best way to help with the crowding at night at MK is fairly simple: offer more enticing night time entertainment at the other parks and keep them open later. A big reason MK gets larger crowds is because people hop to there after other parks or simply plan any late nights that they anticipate at WDW to be spent there. If other parks were open later, it would lessen the number of guests who end the night at MK instead.
And it will be interesting to see how MK is during Wishes once the MSEP ends so there isn't a night parade, while simultaneously RoL opens and DHS continues to have two large evening shows. We
might see an impact on crowding for the MK fireworks, though I think the larger impact will be once Pandora opens along with RoL running.