The point is that they couldn't. You could not charge for FP+ under the old model to all guests, AND still keep the preference for selection by on property guests. No one is going to pay the same price for FP+ if other people are getting a chance to select 60 days out and you only get a chance to select 30 days out, or 10 days out if you booked last minute, or day of if you are making a day trip.
Also Genie plus is sold as a package, not as individual LL selections, which is closer to what FP+ was. Sure if I am picking at 60days out and I have the choice of everything available, I might pay for 3 FP to get lets say 7D, SP, and Splash. But if I am again picking after all the resort guests have already scooped up the prime FP's I am not going to pay for Dumbo, carpets, and Little Mermaid (ok i might for Little Mermaid as it is a family favorite of ours.)
The current system allows you to sell Genie plus to EVERYONE. Late bookers, on site, off site, day of guest, the $15 purchase of Genie plus is equally valuable, giving you the bigger market. And if the point is to generate revenue and NOT incentivize on property stays, that is what your looking for. The maximum number of people possible getting enough value that they feel the $15 is worth it (or required.) Not that it doesn't give some people as good a deal as FP+ did.