If you assume a Moore's Law-like increase in physical qubit capacity each year, it will take slightly under 20 years. That also assumes the rate of decoherence

If you assume a Moore's Law-like increase in physical qubit capacity each year, it will take slightly under 20 years. That also assumes the rate of decoherence decreases commensurately. The reason you get pushback is because Moore's Law exists within a historical context in which it was actually plausible for exponential increases to occur each year, year after year. The field of quantum computing is so nascent that such a context is utterly alien to it. We simply don't have the economies of scale, engineering capabilities nor even theoretical groundwork required to achieve and sustain those kinds of improvements. The other reason is because transistors and qubits are not directly comparable, and you shouldn't try to infer the growth trajectory of one from the other's. So to answer your question directly - it's not nonsensical at all to measure and forecast the rate of physical qubit capacity increase. The report I cited in another comment does exactly that, which is how you can derive that 20 year estimate I made. But we don't have any evidence those kinds of annual doublings will be achievable anytime soon, so most of it comes down to educated guessing.