I've never done ADRs. First of all, I never know what type of food I'm in the mood for, or what time I want to eat. Secondly, large meals and amusement park rides don't generally go together, at least for me. So, we eat at TS restaurants occasionally, but it's not a huge deal. If there's one nearby that we've wanted to try, and the wait isn't bad, we'll eat there. If not, we get CS somewhere else. It just isn't that big of a deal.
I think FP+ "reservations" will be similar, in a "self-fulfilling prophecy" sort of way. If everybody thinks that you must book your FP+ precisely at the beginning of the window, and you can never change them, then they will do so. This will lock up all the fastpasses, and nobody will ever get to change them.
Contrarily, if people don't want to plan their entire day out months in advance, then there will be plenty of FP+ available for whenever you want to book. If people want to change their FP+ the day they arrive or while they're in the park, there will constantly be FP+ opening up.
So, despite Disney's elaborate planning, the success and flexibility of FP+ largely depends on whether people feel like FP+ need to be hoarded or not.
Unfortunately, it looks like most DISers are going to be hoarders.