If they just wanted to data mine, they could have set up Legacy FP machines to read Magicbands. That's all the change that would have been necessary. It would have been less complicated as well, because they would not have had to set up the reservation system the ability to change FP, etc.
I think the idea of 'gateway drug' is possible... encourage people who don't know much about planning for the parks to make reservations, then get them into the system. But I'm not sure how much impact this would have overall. I've also seen the theory that if people plan months in advance, they are better able to staff the parks with the appropriate amount of CMs.
I've seen people speculate that the FP system was changed to take away the 'power' from super-planners (like people on this site) who got 7+ FP per day, and give it to people who sleep late, and 'clueless' first timers. Since these people are the majority of visitors, FP+ is theoretically improving the experience for them.
I'd love for someone who understands the way executives think, and the way corporate decisions are made, to comment on this. I've asked Jim Hill a few times, but he seems to be avoiding the subject, remaining incredibly mum on such a major chance to the Disney infrastructure.