The proofs for the security security of most of the methods of public/private key systems are weaker than "you can't reverse this hash function".
They are still robust in the face of computers that may physically exist in foreseeable futures. Elliptic Curve Encryption was adopted for being post-quantum twenty years ago and more work since then I haven't followed.
The person I am arguing with is imagining a future pessimistic beyond what most would consider reasonable.
