[neltnerb]

Why do you think so many qubits would be needed in order for it to be serious? The article quotes quantum computer researchers as saying 50-100 qubits could perform computations that are intractable on any classical computer. I'm inclined to believe them, the state space grows way faster than linearly as you entangle more particles.

[Strilanc]

To encode 100 logical error-corrected qubits, you need on the order of 100K to 1M physical qubits. People mix up the two types all the time.

[efangs]

It's true that at 50-100 physical (not logical) qubits, it's no longer possible to simulate classically. That does not mean that they are useful.

As other people pointed out, you may need a factor of 100-1000 more physical qubits to get as many logical qubits.