Exactly! Take it from someone who lived in Asia for years, including Tokyo, with the distance between all these parks and the amount of people who live within just a few hours from all these destinations, there wont be any problems having seperate audiences with each park. HKDL hasnt affected any of Tokyo's attendance and if anything most Chinese still travel to that park vs HKDL because HK sucks! I will say though Hong Kong cant be happy about this. Regardless, the entire point of HKDL was to get mainlanders to the park (who needs reentry visas to even go) and now with SDL, true it wont affect people living in the lower provinces near Hong Kong too much, but it WILL curtail mainland visitors regardless...especially if it turns out to be the better park.
But its not that difficult to get a Chinese VISA for American tourists so I dont think it will stop the hardcore bunch. Its about $75 for Americans and it takes 1-2 days to get and you can travel for a month in China. True HK is open for Visas but the mainland isnt much of a pain. And who knows they might even lower the restrictions to get people to travel to the park. At the end of the day, its all about tourism, so making a high visa fee isnt going to drive tourists to the park when every other Disney park is easy for Americans and Europeans to travel to.