Quantum computers will not solve the discrete log or factor a large prime(*) in our lifetimes.

• faster than a contemporaneous classical computer, for a practical key size and algorithm with full iterations

If they do then I will eat my hat.

Based on what? Field has been progressing steadily for a decade.

which are append-only distributed ledgers

Lol, couldn't bring themselves to say "blockchain".

On one hand yeah there's been progress, but on the other we're still not anywhere close to having any useful computation done on them.
It's difficult to predict if the technology will come close enough for a state actor like the NSA to build something capable of attacking current modern encryption. I suspect if it does, they'll be the first to build one and we still won't know if it's possible.

Small numbers factored. Error correcting progressing, material science and they just got qbits needed to crack RSA in theory under 100k. And yes NSA is expected to build one early. And China and GHCQ wanted to start on one a few years ago.