The world may have had two more centuries for that, and won't have been prepared all the same.

Sure, Russia too often reduces the problem to previously unsolved one. This is definitely a strong habit of hers. But those whom history hath joined together at the hip, like Russia and Ukraine happened to become, are notoriously hard to put asunder. See, e.g., examples of dynastic cycles starting from Chinese antiquity (Qin Empire cleavage, Chu-Han war, Han Empire restructuring by Liu Bang), currently (not quite) ending in 1949.

I think even the least competent Western analyst can’t be expected to seriously consider possible future Chinese collapse as final.

Ditto Russia. It can exist only under the imperial aegis, which is not good or bad per se, but simply most logical from the PoV of its historical evolution, marked with oscillations between cosmopolitical and autarkic attractors.