If/when they sell more FPs I think they will take away from the pool already there to make people buy them.
It's going to be very interesting how Disney chooses to implement paid Fastpasses.
Clearly Fastpasses will need to come from somewhere (presuming they are already maxing out availability), so would it be from the pre-selection pool or the same day availability pool? (I personally believe both existing pools will be reduced - and you thought some Fastpasses were impossible to get at 60 days now? The future may be bleak.)
We know they have been playing games with reducing capacity on various rides to save on labor costs; would they ratchet up capacity in order to increase the pool and offer more Fastpass availability?
And I'm convinced paid options will be offered through the Genie as an instant redemption, with corresponding variable pricing to boot. That $100 per instant Fastpass may not look so steep when it's the middle of the afternoon, it's hotter than hell, your two kids are crying, and you're just putting it on your credit card anyway.