Oh, you mean like how Iraq bought all that good modern stuff in the 1980s?
Hint: it doesn't matter what they can buy. It matters what they can actually use in a war.
The Russians and Chinese have decent air and ADA technology and nuke capacity.
Of the three statements in that sentence, only the last is true.
There are a number of ways to prevent air superiority.
Using nukes is not a viable option, unless Russia or China are really, really intent on committing national suicide. Yes, there are a number of ways to prevent air superiority exclusive of nukes--but there are a number of ways to defeat those measures, too.
BTW, the only plausible "China" scenario I see has us aligned with Russia, and my money's on the "Yangtze Station" scenario (we re-establish gunboat patrols on the Yangtze River to deal with the various local bandits and warlords).