What percentage of their current audience would be classified as 'hardcore' Universal fans?
The Harry Potter thing is a little weird to me, because I generally don't see the audience on Harry Potter growing much more past the current base (with the movies fizzling out). And with the old audience, they will have already seen the attractions in the other two parks by the time the third opens. I suppose the third could generate some renewed interest in the other two attractions.
I agree and this is the real question, whether the demographics between Disney and Universal are disparate enough that success or failure at one doesn't really impact the other. Hence the concern that the people most likely to go to Epic Universe are the ones already going to a Universal park to begin with.
There is, to be sure, a segment of the audience that would be classified as more general theme park fans that might switch spending from Disney to Universal, but I think they are generally a small group. At least where non-locals are concerned.
For locals it's a slightly different question that relies more on pricing. If Universal stays intent on keeping pricing low to encourage more local visitors, you could see a shift in local visitors prioritizing Epic Universe over WDW. Generally though, I'm thinking of the people who make 10+ trips to theme parks a year, shifting more to Universal. If Universal ends up wanting to chase more revenue though (to pay for the new park) that might not be where they want to go. They could end up in the same place as Disney, thinking the locals are just unfavorable.