It certainly has that potential - especially if they make it a cheap clone like Hong Kong. However, if they take the park in a new direction and it is truly unique, it won't hurt any of the other Disney parks. It might even have the opposite effect and help spur new growth, technology, and ideas for the US parks.
I'm not nearly as worried about the Shanghai park as I was (and am) about the Hong Kong park.
Think of Shanghai Disneyland as a regional park. Few that visit the Shanghai park will ever have the means to visit the US parks, and few in the US will ever consider visiting Shanghai. They just don't compete with each other in a direct way as perhaps Disneyland and Disney World do.