If all guests to Disney parks would want to stay onsite then Disney would have a serious capacity problem. I would think that a good portion of visitors to the Orlando area staying at the regions estimated 150,000 rooms visit WDW. If FP+ is going to be an advantage to those groups that stay on site then Disney needs to build more rooms - either that or the price of a Disney hotel will go up dramatically as demand increases for perks such as FP+.
The problem goes back to ride capacity at 3 of WDW's theme parks.
Excluding the Swan & Dolphin (which it seems Disney intends to lump in with offsite guests) and Shades of Green (which is no longer owned by Disney), WDW has about 28,000 rooms.
With that number of rooms, Disney is once again being forced into offering tiered attractions at Epcot and DHS. For example, either Soarin' or Test Track but not both.
Building more onsite rooms without building more attractions means even more tiered attractions, devaluing FastPass+ further.
In order for FP+ to work, Disney needs to be able to offer at least 3 FP+ selections to its onsite guests
that they actually value.
Telling me that one of my FP+ selections is Figment is not going to inspire me to extend my onsite vacation.
And telling me that for my $500/night room at the Grand Floridian I have to pick between Soarin' or Test Track makes me wonder why bother.
Why not stay at one of Universal's Deluxe Resorts (that are less expensive) and get
unlimited Express Pass for 2 theme parks that have lots of E-ticket attractions.
Oh and, by the way, Universal is building lots of new, exciting attractions.
Pre or post FP+, it still comes down to the fact that WDW needs more
good attractions at 3 of its theme parks.