Not sure why you're getting downvoted; I do think you're bringing up a valid point against my original comment: DH is susceptible to Shor's algorithm, too. That being said, the question is how long is it going to take to break a single DH key once we have adequate quantum computers? If it's in the order of, say, a couple months to a year, a ratchet algorithm will still protect privacy in the grand scheme of things, as it won't be feasible to decrypt more than a couple select messages per computer per year. Sure, quantum computers might improve, get cheaper and everything but on what timescale? It's not unlikely that that'll take many years and by that time no one might care about your private messages of today anymore and we might have established a new set of cryptographic schemes that are quantum-resistent.

Not them but you are replying on a thread talking about how it isn't safe in the longer future. That context was already built.

Quantum computers absolutely exist and are commercially available. They're just not very useful at the moment.

It get exponentially difficult to add more qubits so it's not a given that we will be able to build one large enough to be a real threat to modern cryptography.

“Quantum computers that break diffie hellman as easily as RSA”, where “easily” means “not at all”, do exist.