100% speculation, of course...
I don't think Disney will resort to canceling reservations outright. The current policy is probably the most workable; forcing people to keep their exact travel dates and not giving them any say in the new resort destination if they want to keep rates/discounts. That's sort of a semi-hard line stance for their least profitable business.
And they'll only do that for people with current reservations. When things reopen for new bookings, choice of hotels, rates and availability will be adjusted accordingly.
Disney will open as many hotels as they need to satisfy guests, while not overflowing the parks.
Despite the theme parks' lowered capacity, I don't expect they'll have to cancel hotel reservations entirely. Looking at the DVC reservation system, there is a LOT of availability throughout the summer. Resorts like Beach Club, Grand Floridian and Poly still have long stretches of availability in multiple room categories. And these are the people who have typically been to WDW the most, and will be most forgiving of having to miss fireworks, parades and character greets during their 2020 trip(s).
On that basis, I think cash reservations are going to be down considerably. It will mostly be die-hards willing to pay premium rates for a watered-down experience.
Things start to pick-up in late September and into October. But let's wait and see what's happening with the Food & Wine fest, Halloween decor and the virus. Demand to visit is going to ebb and flow relative to the virus situation. If things continue to flare up and park capacity is still low, more people will cancel. If the virus is under control and capacity restrictions are relaxed, they'll have room for more guests.