When quantum computers break existing algorithms, we do not need to resort to quantum key exchange, but we can make the switch to post-quantum crypto, as also indicated by the other comment[0].

However, switching to the next generation of crypto only after the current generation is broken is not a solution. Much data needs to be kept secret for a long time. Internet traffic is currently being stored so that when the current crypto is broken all data can be decrypted retroactively.

[0]: https://news.ycombinator.com/item?id=18327175