It is totally reasonable to predict a quantum computing winter, but all the supporting arguments here seem to be about how "hard-headed engineers" know best.
> So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be at least 2^1000, which is to say about 10^300. That’s a very big number indeed.
Yes this is the whole point: if it wasn't this big, we could just simulate it on a regular computer.
> To repeat: A useful quantum computer needs to process a set of continuous parameters that is larger than the number of subatomic particles in the observable universe.
Yep.
> it’s absolutely unimaginable how to keep errors under control for the 10^300 continuous parameters that must be processed by a useful quantum computer.
I do imagine it, every day. But I'm a professional, so maybe that doesn't count?
> How many physical qubits would be required for each logical qubit? No one really knows...
There are plenty of such estimates around. Obviously it depends on the noise rate of each qubit.
> all of the assumptions that theorists make about the preparation of qubits into a given state, the operation of the quantum gates, the reliability of the measurements, and so forth, cannot be fulfilled exactly. They can only be approached with some limited precision. So, the real question is: What precision is required? ... Amazingly, not only are there no clear answers to these crucial questions, but they were never even discussed!
There are boatloads of papers (and conferences) that discuss exactly this. Obviously it is a vital question to answer!