I think the reason the original FP didn't work for Disney is that, for some reason, they thought people would shop and eat while in the virtual queue instead of just get on shorter standby lines and wait in two queues at once.
I'm not sure what the FP+ goal was other than maybe to encourage onsite hotel stays with the extra window. Surely there were cheaper ways to promote onsite stays.
The issue with moving to a paid version (from a guest standpoint) is that in order for it to have any value, it has to be ridiculously priced or extremely limited. If it is not limited by price or inventory availability, then it will not be of benefit and will make the standby lines unacceptable.
As we can see with no FP, even with the COVID capacity restrictions the standby lines are not exactly short so just going to no FP of any kind doesn't help the wait time issue.