I think primarily it will depend on the expansion plan for Hong Kong Disneyland. As Shanghai is breathing down it's back, it will need to seriously look at investing in some unique yet popular additions to continue to encourage Mainland visitors. Also don't forget you have Disneyland Tokyo (and DisneySea) in the mix of Asian Disney theme parks which Hong Kong and Shanghai will also have to compete with, although I am pretty sure Shanghai will sustain itself on local visitors given China's population.
From what I hear Shanghai will be unique in that it won't have the traditional 'Main Street', immediately offering some differences in the parks. Similarly they can't add Toy Story Land, Grizzly Gulch or Mystic Point for at least another 5 years, which in the short term will allow the 2 parks to have enough differences to give people choice. In some ways Hong Kong having the smallest Disneyland may be advantageous to those with younger children or those who want a 'cut down' or 'quieter' Disneyland experience.
However, having said that, Hong Kong Disneyland should not rely on Mainland Chinese alone and could refocus their advertising to the Asia-Pacific region. Marketed correctly it would certainly gain more tourists from Australia and New Zealand as it is a more cost-effective holiday from that part of the world than California. Add to that the rest of South East Asia and there is still an ample pool of potential visitors to the Hong Kong park.
Shanghai may struggle to draw the international crowd (although I don't think it will need international visitors to sustain itself) as unfortunately some Mainland Chinese have contributed to developing a bad reputation at theme parks like Hong Kong Disneyland and Ocean Park, as well as the concerns about language access (which Hong Kong Disneyland also has).Â
Obviously the drawcard to Shanghai for Mainland Chinese will be the ease of access - ie no visa required, able to use RMB, focus on Mandarin language and simplified Chinese; however I think you might find those who come to Hong Kong are not only here for Disneyland, but also to indulge in shopping and food, which perhaps targets a different group of Mainland Chinese...
To say Hong Kong Disneyland will not be impacted would be silly, as of course it will be. Let's just hope they have enough of a development plan in place to compete and complement Shanghai when it opens. It would be sad and illogical to see a Disneyland die because it's sister theme park destroys it. I really hope the Asian Disney parks work in collaboration with each other to provide a unique yet inviting experience to lure everyone to all the parks regardless of where we come from.