Based on watching investment trends and on my own personal experiences, I would say that getting another partner involved COULD make for a better experience.
If Disney had 100% control of Hong Kong, it probably would not have build the three mini lands and not be in the midst of a larger expansion. The government is driving the expansion. Disney is still largely averse to spending money when it feels it doesn't have to, and any partner who 1) is pushing for expansion, and 2) is putting up a good portion of the bill will result in more attractions being built vs if Disney was building and investing all by itself.
How much control does Disney have over the Tokyo parks? 0%. And those parks are easily the best Disney parks in the world: everything is quality, they're staffed to ridiculous levels, and at half the admission price of the US parks (although that is roughly true for all of the Disney parks in Asia).
Until very, very recently, the parks that were getting more investment, and better investment, were those properties in which Disney worked with another partner.
And I can tell you that the Asian parks are all much more pleasant to experience right now, even in heat and humidity and epic crowd levels, vs the US parks.
Now in fairness, this didn't work so well in Paris, but in my (very limited) understanding of what happened there, I believe that Disney's overbuilding of hotel rooms and crippling licensing fees crippled that association's efforts. But someone with more background on that should expound on that.
Ultimately I think that at this stage it would be beneficial for the parks to have a partner. However, WDW is so lucrative for them that I doubt they'd ever seek one. Sure, they like free money for essentially doing nothing (see Tokyo), but if they were to give up ownership, that would mean less money coming directly into their pockets. I think it's unlikely, even though it would likely result in better parks.