I think that DHS is quoted as having around 8,000,000 guests which averages out to about 21,000 guests per day. At 10,000,000, it is around 27,000/day. However, both of these don't take into account when the park is at max during Christmas week, spring break, etc. or if Disney inflates the number by how they count visitors. Still, the 30K/day wasn't too far off.
We rode the Navi River Journey this year and the line was 60 minutes. However, it was one of those where we exited thinking, "Never again unless the line is 20 minutes long". Slink Dog was a 75 minute wait. We enjoyed it but waiting that long for a ride isn't easy. I just can't imagine waiting two hours plus, which is what Flight of Passage pulls on somewhat busy days.
The only thing that could help is that there will only be so much demand. If Disney builds all four parks out with rides/attractions, capacity goes up and the crowds "should" be spread across all four parks. Right now, MK has almost twice as many visitors as each of the other three parks. Regarding additional visitors, I'm not sure how much upward the demand would shift. At the end of the day, it is an expensive vacation that a lot of people either cannot afford or choose not to afford.