Quite literally put the money either into building up their own IP, or shoring up existing institutions (and rebuilding from said event).
Disney is a great showpiece, but in the end the park as planned is... rather western and counterintuitive to the government's Emiratization programs. They have worked incredibly hard (including sending students to Rosen, where i went to undergrad/grad school, to learn theme park, entertainment, event, and hospitality, management) to uplift their domestic workforce, it wouldn't surprise me to see them want to have something in Yas that is much more culturally adapted for the local market. It's not a WDW-sized resort inland (which was always my preferred plan), it's a smaller version (when you take in the back of house and resort areas) of Hong Kong... Not ideal to say the least, with almost no expansion options. Could they go forward? Sure, but it's not in their best interest to do so except to be able to point at the sign and say to Saudi, Qatar, or Bahrain "We have Disney, you don't". Which is effectively what they're doing. Building up their own IP, or using the rich history of the surrounding region to create something much more unique? That's a moneymaker... And something people will fly to see if sone right, instead of a small castle park. I hope, but do not know, if that was ever thought of when deciding what to do with the space.
And if they don't? They can shore up the existing Yas Island developments with that money. Expand WB, SeaWorld, and Ferrari World, among others (they have a water park for example that is hardly talked about which could be even more-so world class if they added a few things), or pay for the Sphere they want to build. Or do anything of a dozen other things that are frankly a better investment in the short-to-medium term (I could write a whole 50-page paper on this to be honest). Or even take that bunch of money and use it to rebuild from the event that has clearly devastated much of their economy (and could continue to do so without a deal) to the point that the money Miral was going to spend might be redirected anyway to pay for such repairs and upgrades that are considered more essential for the people who live in the UAE vs visit. It's a tough choice, but it's one that might be made for them.