Once large-scale quantum computers exist and can break RSA, of course everyone will switch to a quantum-resistant cryptosystem. Sure, the work that classical computers do to encode and decode will be polynomially higher, but on the other hand, all of the code-breaking advantages of quantum computers will be nullified, and we’ll be stuck trying to explain to the NSA why they should fund simulations of benzene.
Or will we?
One thing we’ve learned from the (semi-)recent hack of stratfor.com (a “subscription-based provider of geopolitical analysis”) is that even though the importance of salts has been known since at least the days of the Enigma machine, it doesn’t mean people will actually use best practices, and stop storing things like credit card numbers in the clear.
So don’t worry: even after “post-quantum” crypto has been developed, and large-scale quantum computers are common, there should be plenty of RSA around to crack.