To me, I think Osaka will be able to sustain itself more-so than I would even argue Singapore, due to that by the time the park opens, it will be the 2020 Tokyo Olympics.
But, we are also arguing on that it's somewhat like the United States in the terms of flow, as Beijing will be in the middle-point of geography between Singapore and Osaka.
It's just how I see it, and that should be perfectly fine as many can have different opinions and outlooks on what we may see. Will Beijing take away from Osaka? Probably. Will Beijing take away from Singapore? Probably, and we should look at it from an overall perspective.
I haven't seen anything on attendance drops of Tokyo Disney due to Shanghai, and I can't see it having that much of an impact for Osaka when Beijing opens.