Nexus is using 571bit priv keys, still as i believe it is using elliptic curve cryptography but having a greater bit length means it would require much more qbits to crack than bitcoin for example. Theres no such thing as quantum proof only quantum resistant cryptography.
 
Also regarding the argument that bitcoin can simply fork to a new algo, no one can even agree on how to solve the scaling issue with bitcoin i wouldn't bet on a hard fork to change anything in bitcoin being adopted in a short span of time. The time in which it took to reach consensus would leave any priv key used more than once vulnerable.