I think this is the key to the whole system.
I'm not quite sure that most people won't want to schedule their whole day. After all, why else to people get up at ungodly hours for rope drops, religiously study touring plans, hoard fastpasses? These are all plans.
However, they're FLEXIBLE plans. If I know I'll want to spend the morning at AK and the evening at Epcot, I won't mind scheduling several FP+ in the morning at AK and several more in the evening for Epcot. But I want the FLEXIBILITY to change my mind based on weather, crowds, or just a whim. If I'm "stuck" with those parks from 120+ days out, I'll be mad. But if I can whip out my phone halfway through the day and say "nah, let's do MK tonight instead" and still have the ability to move my fastpasses over to the new park, then it's fine.
So, I don't think that
MDE and FP+ are inherently "too rigid". I think it will all come down to how flexible the system is, and we just don't know about that yet.