It didn't work because they promised that everyone would have the ability to book 3 fastpass+ selections per day in advance. That was the part that broke it. There was not enough capacity for that to be realistic.
Genie+ is different in that you (currently) purchase day-of and make day-of
Lightning Lane selections only. That is no small difference. Also, a lot of people buy Genie+, but I'm going to guess its utilization rate is lower than Fastpass+ simply because it is not free.
So I would argue Disney is not (currently) charging people twice for the broken FP+ system. Now, if they make changes and allow advance purchase of G+ with pre-selection of a # of LL for each day of your trip, THEN I would agree with you about "charging people twice for that experience that didn't work to begin with".