dstarr3 :
We already have quantum computers that work.
There are countless of promising future technologies that have been proven in labs but never got commercialized because no economically viable way was found to manufacture products based on them or by the time that viable manufacturing became viable, something else came along and rendered them useless.
Nuclear fusion is one example of technology that scientists originally thought would take only 20-30 years to research and 50 years later, a commercially viable implementation is still elusive. For quantum computers, the output is a statistical distribution of possible results and the amount of uncertainty increases with every qubit added, so you have the challenges of packing more qubits together, reducing the amount of output noise, finding suitable room-temperature materials to make them from, finding ways to get data in and out of them, etc
I get a feeling that the more scientists research quantum computing, they'll discover that there is even more that they still don't know about and that commercially viable quantum computing is further away than they thought.