Supersonic doesn't equate to invincible. Ultralow flying aircraft going at supersonic speeds aren't invulnerable either.
Slower speeds allows for better terrain hugging (compare fast jet and helicopters) and avoid specific defences.
Hey man,don't forget that Brahmos was primarily developed as an advanced anti ship missile that can fly low and defeat the heavy defences of warships.The block2 can fly at an altitude of 8 meters with help of a laser altmeter which is actualy terrain 'licking'rather than huging.Besides its high speed does make it near invulnerable because we are here talking about pakistan not china.Pakistan could not even detect the slow moving black hawks,let alone detect and destroy a mach 3 capable,terrain huging cruise missile.
REGARDS....