The whole My Disney Experience web and apps cloudification of electronic info and reservations including FastPass+. Supposedly a $2B price tag for it (think of the infrastructure needed to have almost every ride in every park have FP tapstiles and a separate FP queue).
On Disney's end, they believed FP+ would move people around enough to give guests a chance to ride the big E-Ticket rides without waiting in several 2 hour queues, and thus solve their overcrowding problem.
The real problem, however, was the unending increase in guests year after year which surpassed MK's ability to give everyone a good experience. And so, FP+ didn't accomplish what Disney hoped for, and so Disney tried to find other ways to deal with overcrowding, like raising prices and discounting off-season, and making the other parks more attractive with more attractions.
Meanwhile, FP+ is being blamed for over crowding rather than crowds being over capacity. Which, in a way, it is in that it stalled Disney's opportunity to deal with the real reason of overcrowding, however, once you have too many people in the park, FP+ isn't the cause of long lines... too many people are.
Anyhoo... personally, with the ability to manage hotels, restaurants, FP, mobile ordering, maps, info, etc... I think Next Gen is great despite it not accomplishing what Disney wanted.
Disney still has an over capacity problem in MK still, tho (outside of a pandemic)...