Yea this is the point I was making quite a while ago now when debating off-site pre booking.
If the entire point of FP+ is as stated 'To "lock" people into the WDW vacation, to discourage them from going elsewhere in order to ensure that WDW is getting more of the Orlando vacation dollar' then Off-site pre-booking has to take place.
As noted, I would imagine the VAST majority of onsite guests do not have access to a vehicle. Let me know if I am wrong by all means. But the provision of transit to and from WDW and around WDW already Locks" most onsite guests to WDW for just about everything. While its true that crowd management and line management MIGHT increase spending of onsite guests through FP+ ... I am still waiting to see if this actually pans out or backfires.
The people who they NEED to lock in, to prevent them from going elsewhere with their $$$ are the offsite guests. They are the ones that can choose to go elsewhere for meals, or for souvenirs, they can spend more time at "another" park/attraction etc. So they are really the ones that need to be locked in.
MBs will somewhat increase the spending of onsite guests (we know this from a ton of social research) but I think one of the main foci of FP+ has to be locking in those offsite guests. I think again this is part of the reason you can pre-book at 60 days, heck they might even increase this further out (unless this number is already supported by research, which it probably is) if they are finding the average offsite guest "decides" what they are doing before 60 days.
Its going to be interesting to see how this develops, even if I don't like the system I do still find it interesting, maybe it will go in a direction I like more, maybe not ...
Should also say this might still be combined with some advantage to onsite guests, 60 +10 for onsite, or even 80 day for onsite 60 for off (depending when that magic "decision day" is ... 3 FPs for offsite and 4,5,6 for onsite... Of course, with both of these we get into issues of capacity, and again $$$.
If onsite guests truly are locked in, and offsite guests really aren't, from a Disney perspective it might make MORE sense to give OFFSITE people an advantage over onsite .... OMG, seriously right ??? (it depends how much this would effect onsite bookings of course) If I, as an onsite guest, am already locked in it doesn't matter if I have a 10am Test Track and then a 6pm Soarin FP .... because chances are, I am not going anywhere anyway, I will spend my money at WDW regardless. BUT, if you could give an offsiter a headliner just before lunch, and at a time later in the afternoon probably a good enough distance from lunch that they are likely to buy some dinner at the park as well .... BAM !!! That's a revenue generator right there ....
Heck this system could even do this automatically ... lets say I FP TT at like 1pm, and via those wonderful magic bands WDW can detect I ate lunch at a QS at 12 and I have no onsite reservations for that night ... I could automatically give me a prompt with a Soarin FP+ reservation availability at ... 6pm ? 7pm ? With research it could really nail this down to a time differential that limits the chance I go offsite to grab a bit and then come back, but increases the likelihood that I get too hungry, cave, and but our dinner onsite as well ... Ooooooh the magic of technology. Of course, I as a consumer am not really a fan of this, because me, they aren't going to offer me any of this, I will have my ADRs all selected