I suspect some people are guessing that a multipolar world will be less interventionist (since the great powers might block each other from interfering too much) and therefore adventures like the Iraq war are less likely to happen again. This might arguably make the world more peaceful, though it might allow civil wars to continue for longer.

No idea if it's true or will work out that way, but it's one hypothesis I think is out there.

Sure, the downside of China's rise is that authoritarian states have more legitimacy, but it's not like the western powers did much about those states back when they were the hegemons, so I'm not sure if the argument carries much weight either.