You are assuming that non-planners want a plan in the first place - which we don't. I go on vacation to relax. My every day life, especially at work, is planned beyond belief. When I go on vacation, I want no plans. If I want to lie in bed and order room service all day, I will. If I want to go to a park and just people watch, I will. I will eat when I get hungry, and I will choose what I want to eat based on what I feel like eating when I'm hungry. I often just wander out to the bus stop and get on whatever bus comes first. Sometimes I'll take a bus to a park and see a bus to a more interesting place when I arrive, and jump on that other bus. It may not be anyone else's ideal way to vacation at WDW, but it's my way and it works for me.
That being said, I seldom use FP and I seldom make ADRs, because both require me to be at a certain place at a certain time - and that's not how I vacation. If I wander by an attraction and the standby line is reasonable, I get in it. If it's not, I'll catch it another time. I'm not big on thrill rides, so many of the hard to FP attractions I am not interested in anyway.
Personally, I feel that the planners will enjoy this change more than the non-planners, because they will be able to plan another aspect of their vacation way ahead of time. But for non-planners who want to ride the thrill rides, it may not work out as well as the current FP because I don't feel you can be as spontaneous with FP+.
But until it goes live and we see exactly what impact it has, we don't really know for sure what will happen. It's all just guesswork right now.