I think you're missing the point. New guests are not going to return because of FP+. FP+ is just a tool. It's like saying guests will return because the bathrooms were clean. Well no. Of course not. Clean bathrooms are one nice feature of WDW, but it's not why you go back. You go back because you LOVE DISNEY WORLD. With all you experienced - the clean bathroom, the dirty bathroom, the CM that was nice, the CM that was mean, the time you waited an hour and the ride broke down and you got shafted, and the time FP+ worked for you or didn't work for you, you go back because ultimately Disney has what you want -- the characters, the rides, the movies, the connection, the love, the magic, whatever.
FP+ is just a tool for Disney to manage crowds and optimize profit -- given that these crowds are coming to Disney in droves.
Right, as I mentioned in an earlier post, it's the overall experience that will determine whether or not people decide to return to WDW. But you wrote "I believe Disney is okay losing a few to gain a bunch." How will FP+ gain them a bunch of visitors? One useful fast pass per day, coupled with longer overall wait times (probably) is going to gain them a bunch? No.
Again, FP+ is the marketing face of this data mining experiment. Yes, I call it an experiment because it may well be an overall money loser for Disney, thanks to continuing cost overruns, growing pains, and assorted tech glitches and screwups. Hell, after all these years, Disney's websites STILL have issues!
Let's do a little breakdown of what FP+ does and does not mean.
It DOES mean that many more people are using fast passes.
It DOES NOT mean that people are coming to WDW to experience the joy of FP+. People go to theme parks to experience everything those parks have to offer. That won't change because of a fancier reservation system.
It MIGHT mean that 50% of guests will have a slightly improved overall experience, because they got on a headliner faster than they would have in the past. Again, that depends on the overall net effect of FP+, which may well be negative.
It DOES NOT mean that people will change their reasons for deciding whether or not to come to WDW. First-timers tend to come to WDW because of its fame and world-wide reputation. Returning guests are more likely to plan their trip around the opening of new attractions.
Which bring us full circle, because to me, that is the real big picture here. That $1.5 billion could have, instead, bought a lot of great new attractions...