But as far as I know, quantum computers are just an abstract theoretical concept, right? I don't think there's any working ones outside of very limited laboratory tests. Certainly nothing practical in the sense of being able to do ordinary everyday computing tasks. So I'm not overly worried about that for the time being.

I admit it is a concern in the long-term. Probably the entire field of cryptography (and by extension cryptocurrencies) will have to change in some fundamental way if / when the quantum revolution finally comes.