In theory if large scale quantum computing becomes practical then it might become possible to decrypt traffic encrypted using certain public key algorithms. Basically by using Shor's algorithm to factor large numbers. But there's no guarantee that will ever work in practice, and even if it does there are alternative encryption algorithms which are resistant to quantum computing attacks. It's also not easy to identify and exfiltrate high bandwidth data streams even if you compromise a piece of network equipment.

If Chinese intelligence services want information on a certain politician or business leader they're more likely to skip the decryption nonsense and just recruit one of the target's associates as a spy.