Letting more people ride the big new attraction is kind of the point. Putting that aside, longer hours will spread the crowd over a longer period, which would likely reduce lines overall. Without the artificial leveling of fast pass, it will also allow more time for the line length to fluctuate, allowing those who don’t want to wait in the longest line of the day a chance to line up at times when the line would be shorter, such as during fireworks, parades, or near closing time.
More attractions, particularly if they are the kind of SW-themed people-eaters that should have been built, would take some of the attention and crowd pressure off of Rise - this was the initial plan with MMRR, after all.
Most importantly, if people have to physically wait in line for attractions, other areas - and other lines - become less crowded.