I know I am in the minority on this, but I'm one of the people who believes that longer waits are one of the benefits of FP+ whether intended or not -- but I believe that is what Disney wanted. It makes no sense to have a ride like Spaceship Earth as a permanent walk-on, sending out 95% of its seats to run through the attraction completely empty, while other attractions face unbearable crowds.
In an ideal world, there is no such thing as a walk-on except at the very lowest time of year, like early September. A walk-on is wasted ride capacity, which is wasted Disney money, which means our admission money is being wasted. A minimum 5 minute wait vastly reduces that waste, by ensuring a steady supply of riders to fill almost every available seat all throughout the day.
I don't know that Disney needs more attractions, but it does need to replace whatever it eliminates with something more interesting. The real complaint about the longer waits under FP+ is not that a former walk-on now has a steady 10 minute wait, but that the ride itself is not worth riding unless there is no wait. Disney need to replace the attractions recently removed, and then it needs to start replacing existing dull attractions with better ones.
If the 15 worst attractions were replaced by 15 great attractions with the exact same capacity, I doubt anyone would complain even though overall capacity has not grown.