Please consider my earlier posts. WDW is no longer in the theme park business; it's in the hotel and timeshare business.
For Disney, the big money is not going to be made selling FP+ upgrades, which have only a limited appeal. Just like at Universal, most will balk at paying for the upgrade at WDW.
Offering cash FP+ upgrades would hurt their core business. Disney needs to get their $200/night to $1000/night rooms filled. The margins are incredible yet they have thousands of empty rooms during even the busiest season.
Beyond occupying rooms, onsite guests pay Disney's ridiculous price for food, drinks, and merchandise. That's where the money is.
Selling FP+ upgrades only hurts the core business since it provides less incentive to stay onsite.
Think of it this way, if Disney sold FP+, my options would be to rent a 4-bedroom house on I-192 for $150/night, rent a car, eat a lot of my meals offsite, and then pay Disney another $50/person for FP+ or I could let Disney transport me to and from the airport for "free", cram into a $400/night Deluxe Resort, pay Disney $15-40/person per meal, and get "free" FP+.
It's obvious to me which option Disney wants me to choose. In fact, they want me to choose it so much that they are not going to give me the option.
WDW is playing FP+ games because, with 31,000 onsite rooms, they don't have enough ride capacity. WDW needs more rides and fewer rooms. But the big money is in the hotel rooms. It's why Disney continues to build resorts and DVC without significantly growing the theme parks in 15 years.
Since the opening of DAK in 1998, WDW has added over 10,000 rooms!
It's easy to justify the cost of building yet another hotel or DVC, a lot more difficult to justify building another park.
Again, WDW is now in the hotel and timeshare business, not the theme park business.