comparing a Broadway show to a WDW attraction is not comparing like entities.
The "WDW vacation" equivalent of a "NYC vacation" Broadway show is La Nuba. Both are experiences that need to be planned in advance, that you purchase tickets for and are experienced at set times.
Some people who travel to WDW choose to experience La Nuba, those people accept that there is no flexibility if they choose that specific entertainment option. At WDW, just like at NYC, there are PLENTY of other things to do if you prefer flexibility and don't want to be tied down to a specific time or place. The beef that people have with FP+ is that it would cut down on the amount of other things at WDW that don't require pre-planning.
The NYC vacation equivalent of FP+ would be if I had to commit to being at the polar bear exhibit at the Central Park zoo at 10 am three months prior to leaving on vacation. I wouldn't be very keen on that, just like I'm not in love with the idea of hard and fast FP+ times.
Here is the thing, I'm a planner at heart. I love going on to touring plans and making optimized touring plans and I adore spreadsheets and the idea of micro-managing my vacation.
I would LOVE FP+ if I was able to travel solo, the problem is that I have to travel with two small, unpredictable creatures who love nothing more than creating confusion, delay, trouble and mayhem and who always mange to grind my perfectly prepared daily schedules to dust, usually by 10 am.
I love traveling to Disney because no other vacation destination is easier when it comes to traveling with babies, toddlers and preschoolers, however, Disney seems to keep creating policies that are only making it harder and harder. First the CC cancellation policy, then enforcement of FP times, and while neither of those have caused me to throw in the towel yet, they have both made my trips significantly more difficult than they were even just three years ago.
Being limited to three fastpasses a day is a blow, but if FP+ does not allow for dynamic same day changes, I might have to step back from Disney for a few years until the kids get older and its easier to stick to a schedule.
I can see how FP+ may work well in the future, in about 12 years, when the kids are older teenagers who like to sleep in and only want to do thrill rides, but that seems like such a long way off.