So here is my theory - this is based only on my own theories of how/why they would make this work.
I don't think you'll have a problem getting selections day of - maybe not the time you wanted - but I think for all but the busiest periods you won't have a problem.
Why? Because they are limiting how many FPs you can get.. not everyone will be able to get a FP for that attraction because they allocated their choice elsewhere.. their return ratio will be much higher (so the # of passes they give out will be far more true to usage).. and because FP+ will be so much more controlled, they can get far more people actually through the FP+ line.
So my theory is this all will result in much better availability of FP+ slots.
Disney knows they need to keep 'day of..' capacity. They can hold capacity back until day of to meet this need too... or even release new capacity all throughout the day.
These are things that can be balanced if they chose to.
Do you happen to know the capacity of the attractions? At MK, what you suggest might work. They've got a lot of attractions and enough "good" ones that maybe they'll have enough FP+ for most people, if they distribute 1-to-2 "good" FP+ (Think Space Mountain) and 1-to-2 "not so good" FP+ (think SGE).
I just saw an interview with Staggs stating people would get 3 FP+ per day. MK averages a little over 46,600 guests per day. That means they need to create about 140,000 FP+ experiences. Even if an attraction handles 1000 per hour (I think that's low for many attractions), the park is open for an average of 14 hours per day, so a single attraction has the capacity for at least 14,000/day. Assuming they split capacity 50/50 between FP+ line and Standby line, MK needs 20 experiences
for an average day. Still, it does suggest Standby lines might be rough. Al those reports of expected longer Standby lines could very well be true.
Even though the other parks have much lower attendance, they could have a harder time. Let's face it; there aren't that many "must do" attractions at Epcot, DHS, and DAK.
I've read Expedition Everest capacity is 2000 per hour. DAK has much shorter days, closer to 10 hours per day. That's 20,000/day but beyond KS, Dinosaur, Nemo, and Lion King, what else is a "must do" at DAK? Maybe they should have waited for Avatarland before deploying FP+.
I don't think loading will be any faster though. Disney already strives for
optimal load time and nothing in NextGen increases ride capacity.
