It's just the obvious model of computation for our universe if you believe that quantum mechanics, which we've known about since 1926, is true. If quantum computing turns out to be impossible for some deep reason, that's WAY more exciting than if it's merely possible, since it means QM itself is wrong or incomplete that should cause the biggest revolution in physics for a century. QC being possible (and, as a direct result, bitcoin as currently implemented being ultimately insecure) is the "boring," "conservative" possibility by comparison! :-D