I am another "wing it" type person, who rarely used the legacy FP. However, I have found myself using FP+ and liking it. I don't book FP+ for every day of the trip, but I do use it to make sure I get in my very favorite attractions easily at some point during the trip. Most of my trips are solo, so it's a lot easier to find spots at the last minute. So I still wing it for the most part, with just the occasional scheduled FP. There's nothing that says you can't cancel or change the FP if the mood strikes.
I also use it for those days when there will be more than just me touring the parks. For example, I will be there for 10 days in January and for two of those days have a couple people joining me. We have made the FP+ for the two days they will be with me, but I probably won't make any others until I actually arrive.
I understand why people don't like it - it's not for everyone. But the legacy system wasn't either. In my opinion it seems like the people who loved the legacy system hate the new one, and vice versa. The very things I disliked about the old system (having to be at the park at rope drop and running to grab a FP before they ran out, crisscrossing the park multiple times to get the FP and then return for the ride) have been drastically reduced with the new system. However, for those who enjoyed grabbing FP for the same ride over and over, or had a runner system going, it's not so much fun.
For now, though, it's the only game in town as far as WDW goes, and everyone just has to figure out how to make it work best for their family.
I think,based on the increased crowds and FP+, there are guests who are making the most of it by spending their vacation dollars and time elsewhere. I'm seeing quite a few posts from people saying they will take a break from Disney and maybe come back in 5 years.
I don't think FP+ is entirely to blame. I think D has done a wonderful job of advertising, and the parks are always empty in every ad. But the parks are always full and there are no slow times. I really think this, more than FP+ is, going to drive people away. I don't want to spend my vacation standing in lines for food, rides, and bathrooms. Simply put, IMO no vacation is worth that.