"That's not true at all. Scaling quantum computers is exponentially hard - each bit is twice as hard to keep coherent as the last."

I am very interested to know if that is the case. From my perspective, your claim seems like an intuitively obvious application of TANSTAAFL, but there are experts who actually understand quantum physics who think otherwise. Layman's intuition is not to be trusted. Therefore quantum computing remains tantalizingly more plausible to me than, say, cold fusion.