tsnor :
"...they'll never make a computer that can do a megaflop, just think of the failure rate of those vacuum tubes, and the amount of electricity thy used would power a city..."
I realize it's hard to shake the idea that technology can overcome any obstacle, given enough time. But you should consider that there are some limits to physics that no amount time and intelligence can overcome.
Just because you see an analogy in the history of classical digital computers doesn't mean the same potential for improvement in scale and portability exists in quantum computers.
Perhaps you find the prospect that the next 50 years of computing won't be the same march of exponential progress as the last 50 years. It has been an amazing ride. But let's look at another technological trend people once assumed would continue at the same pace: that of power. If you take most of the 1950's and 1960's era predictions of the future, they were based on the assumption that electricity would continue to get cheaper until it was virtually free, and that generation or storage would be increasingly portable. This fueled predictions of jet packs, flying cars, and more elaborate constructions. But that technological trend was quickly slowing and so many of the predictions haven't materialized (or at least not in anything like the fashion and timescale that people imagined).
The thing about technology is that it's hard to predict what people will invent or improve. But you can be sure it won't violate physics. And the physics of quantum entanglement make it
extremely sensitive to interference. That's why I'm relatively certain you'll never have a quantum annealing computer in your home, much less your pocket. And with the ease and flexibility of cloud computing, that's (usually) okay.
I hope that helps, but if you'd prefer to believe that
anything is possible, then just be sure to leave out some milk and cookies for Santa Claus.