> The real problem is going to be all the stashed encrypted data that US and China have stored on each other of a sensitive nature

While there's no doubt they've stored lots of encrypted messages sent by each other, would it really be such a huge catastrophe if those messages were decrypted years after they were sent? Presumably neither side is sending the most sensitive information that has long-term value (like weapons designs) across a channel the other can read, encrypted or not. And the US and Chinese governments losing control of some of their secrets wouldn't necessarily be a net negative for the world anyway. Snowden "decrypted" some of the US's secrets and we are better off for it.

The bigger threat isn't both sides decrypting each other's stored messages, it's one side breaking the other's encryption without them knowing, like what happened with the Enigma in WWII.

Unfortunately the work being done on post quantum cryptography is mostly on the topic of symmetric key agreement (since that is what TLS needs), and cryptocurrencies need digital signature and/ zero knowledge proof systems. There is comparably much less work being done on solving those problems.