The worry isn't whether or not YW will surpass Pokemon outside of Japan, the worry is that YW is currently beating Pokemon in Japan, and that GameFreak, in an attempt to become top dog in it's home region again, will start competing directly with YW for it's target audience, which is very young kids, and that this will lead to a massive dumbing down of the series, more than we've already witnessed.
This is why the success of YW worries people.
