Comment by ISL

If you do this, the enemy's nuclear-weapons services will look in the playbook under "what to do if someone kills the government", see, "launch everything as a counterattack", and press the button.

A key advantage of a hypersonic weapon is the possibility of first-strikes to disable the enemy's retaliation systems before they have the ability to launch more-traditional retaliatory responses. Only submarines are likely to be mostly-immune to them.