Quantum computing will require a huge increase in entropy and changes in methods but we are not going back to the 70s.
Actually, yes we are. One thing that keeps traditional cryptography alive (we're still talking about asymmetric schemes based on large number factorization, correct?) is the fact that the largest quantum unit today (that we know of) is 50 qubits. Make it 5,000 and your SSL session can be decoded in seconds, no matter how good your entropy source is. EC-based is a little bit tougher but not by far.
One technology is there you'll need quantum devices to secure your information. But we're talking about decades timeframe, cryptocurrencies will be long dead by then.