Hypersonic planes may be a viable proposition, but hypersonic missiles are not. According to John Mecklin, the editor of the Bulletin of the Atomic Scientists and Mark Gubrud, a physicist and adjunct professor in the Peace, War and Defense curriculum at the University of North Carolina, they are highly visible, leaving a trail of ionised particles in their trail, and slower than ballistic missiles. They are a weapon system searching for a purpose, as John Mecklin puts it.
They are not even more accurate than ballistic missiles, which was the original idea of such systems. To pinpoint a target they would have to slow down so that a sensor can work, which defeats the object of the exercise.
As Mark Gubrud explains, after WWII, we have been developing faster planes and missiles. The Arms Race really never ended. But there is a limit to how fast these objects can travel and still be effective even with computer-aided design. As yet only about $200 million has been spent on these systems in the US, small fry to what is being spent on other weapons. Be this as it may, arms companies all over the world will benefit from development and production of missiles, which may be easier to detect than ballistic missiles. The public is already been being told that they represent a threat. Mark Gubrud proposes a test ban, which would seem to be a viable solution, as testing these weapons in secret is impossible. Test bans on a range of other new weapon systems should also be established.