I think this depends on Epic more than anything at Disney. If Epic causes a dip in attendance (bigger than Disney finds acceptable) I think they’ll fast track something. Disney is capable of building something quickly, they just don’t so they can spread the costs over multiple quarters.
Another situation where I’d love to be a fly on the boardroom wall, Disney is constantly trying to manage overcrowding so they’ve probably got an “acceptable loss” figure once Epic opens, I just wonder if that’s 500 people a day or 5,000 people a day.
Unfortunately the losses likely won’t be at MK, where they need fewer people.
My guess is (reactionary) Disney will build a ride at HS and AK, likely clones of existing rides from another parks, quickly once Epic opens.