Sea World has kind of morphed into Orlando's version of a Six Flags or Cedar Fair type of park. Same with Busch Gardens.
10 - 15 years ago I'd argue it was a step above a Six Flags or Cedar Fair. I'd imagine that Sea World probably does a higher percentage of local business (Florida residents) than Disney / Universal and also serves as a way that more budget conscious travelers who come to Florida can attend a theme park.
As far as Epic Universe I believe it will draw more people to Orlando but if they go to Disney it'll probably be more of a 1 day MK thing.
I'd imagine MK would get slightly busier and the other 3 parks slightly lighter (in terms of crowds) after Epic opens.
The bigger issue for Disney (and Universal) is a possible recession / temporary drawdown based upon changes in international relations. That drawdown could, temporarily, limit the total pool of travelers willing to spend money on a theme park vacation.
