If you drop a glass cup on the ground, it will break and shatter into pieces. This happens all the time and is consistent with quantum mechanics. But it never happens that a shattered glass cup rearranges itself from the ground into someone's hand as a whole glass cup, even though this is also consistent with quantum mechanics. We see from this example that not everything that is consistent with quantum mechanics is possible.
As far as I know, scalable quantum computing has never been demonstrated either backwards in time or forwards in time. So a fortiori, I would think that this would be good enough evidence to suggest that scalable quantum computing is impossible. Yet, some physicists believe that scalable quantum computing is still possible. Why?