The switch from Legacy Fastpass (where select attractions were able to be reserved on a day-of basis as many times as you could according to the rules) to the Next-Gen technology driven Fastpass+ (where most major attractions and many minor ones now offer FP+ entry, but you’re limited to just three FP+ a day) is nearly as drastic a change as the transition from the famous A-B-C-D-E-Ticket ride coupons to a passport system where one ticket gets you in the park and on every attractions.
My memory is a little hazy, but I don’t recall guests getting so worked up about that switch. That’s because it was largely a switch in the method of accounting in the guest’s vacation ledger. With ride coupons park admission was merely a token charge, the real money was in the coupons. So grandma could take the kids and she would only have to pay a small amount for herself. Single admission changed that model forever. The new model meant Disney got more at the gate, but a savvy guest could work the system by staying from open to close (we called them marathon days) and ride many times more attractions than they could with a coupon book. Plus they wouldn’t be stuck with a bunch of unused A-tickets at the end of the day. So in the end, the ledger balanced for the guest.
A certain camp of Disney Imagineers believe this switch was the worst thing to happen to the parks. That the move away from ride coupons and to a single passport, meant that new attractions couldn’t be cost justified based on coupon purchases, that guest behavior was unleashed and less predictable, that minor attractions suffered in attendance, and that it made more difficult for a family to come and enjoy the park if they had to pay a large chunk up front just to get in. The counter arguments were: that most families on vacation had a set amount to spend and they’d spend it on passports or ride coupons just the same, that allowing guests to experience the park without worrying about buying another E-ticket for Space Mountain provided a better guest experience, and that the real money for Disney was in hotels, food, and souvenirs.