You don't actually have to knock a hypersonic missile off-course, you just need to destabilize enough that it starts to tumble, the atmosphere will take care of the rest. In a vacuum that momentum would be an issue, but air resistance would shred it once it started to tumble. That's why lasers could actually be a halfway decent defense, as all you need to do is slightly unbalance the thing and it'll be shredded by its own momentum.
