Let's consider some rough numbers.
WDW has approximately 4300 DVC, 5600 Deluxe, 7500 Moderate, and 10,600 Value rooms, about 28,000 total. Occupancy rates average around 80%, occupancy is about 3 per room (rough numbers). That comes out to about 67,000 onsite guests/day on an average day. Multiply that by 3 FP+/day and that's about how many FP+ experiences WDW potentially needs to provide each day, about 200,000, for just it's onsite guests on a typical day.
What I don't have a good idea of is what percentage of onsite guests actually visit at least one of the the theme parks each day. However, one of the goals of MM+ is to actually increase this percentage. These onsite guests are the ones spending the big money on WDW's overpriced resorts. Disney wants them in the parks spending more money on WDW's overpriced tickets, overpriced food, and overpriced merchandise. If MM+ succeeds the way Disney wants, the percentage should increase.
Picking on Epcot, there really are only 3 attractions that benefit from FP/FP+: TT, M:S, and Soarin'. Combined, they have a daily capacity during a normal 9-to-9 day of less than 50,000. That's total capacity, not just FP/FP+ capacity. The current ratio of FP-to-Standby capacity allocation is between 2-to-1 to 4-to-1. Let's pick a number in the middle and assume Disney intends to distribute 35,000 FP+ per day for these 3 attractions.
What percentage of onsite guests are going to opt to select Epcot for their one FP+ park? Let's randomly use 1/4th. (I really don't know.) So, 67,000 X 0.25 = 16,750.
If WDW allocates each onsite guest 1 FP+ selection for TT, M:S, or Soarin', then offsite guests have a decent chance at getting a FP+ for one of these, although not great. If Disney decides to allocate 2 FP+ selections to onsite guests, then offsite guests are in trouble.
And remember, this is for an average day. For busy days, offsite guests could be completely out-of-luck.
P.S. A lot will depend on how "smart" onsite guests are. Will they know enough to make their FP+ selections as soon as possible? That, IMHO, is a complete wildcard.