There are several misconceptions about what universal quantum computers could do. One of them is talking about exponential increase of computing power with each new qubit. It just doesn't work like that. This brief talk from Scott Aaronson tackles this issue quite well: https://www.youtube.com/watch?v=JvIbrDR1G_c

While quantum computers threaten some cryptographic schemes, it will just be the end of those schemes, not the end of cryptography in general (see: post-quantum cryptography). As long as we still have problems that are hard to compute and easy to verify, we can have cryptocurrencies. Quantum computers do not give "unlimited computing power" and do not threaten the existence of cryptocurrencies in general.