Can we all agree that ExpressPass worked but FP+ does not? Even that legacy FP worked? I think we can.
And I think that the reason for that is the number of these passes that are in circulation. EP and FP had a fairly limited number; you stay in a UO resort or you know how to use the FP system. Now, literally every single family or group that buys WDW tickets is inundated with the FP+ primer, so the vast majority of WDW park guests have made all 3 FP+ choices for every day, even if they do not know what the ride they "reserved" a time for even is.
With 80% of capacity for many rides going to FP+, the number of people who have this needs to go down to make standby a viable option again. One way to limit is monetarily (as Express Pass does); another way is to require some effort (as legacy FP did). FP+ takes away both of those barriers to entry, and now standby lines are ridiculous.
*I did not post any stats, but Josh at easwdw.com has tables on a fairly regular basis that show the difference in standby line times pre-FP+ and post-FP+ (apologies to mods if citing that site is not okay).