When attacking things the size of country, sensors near the borders can detect the sound of supersonic aircraft and transmit that information at the speed of light to waiting air defense systems. Silence still has some value.
Doesn't really make sense to me. For one, it's not silent, just quieter than you'd normally expect from a supersonic aircraft. It's not stealthy; radar will see it. It can't fly low and still be fast, so it can't hide from radar behind terrain. It's much slower than ballistic missiles, hypersonic glide vehicles, etc.
A high-flying stealth subsonic cruise missile can be as silent as the airliners you see everyday flying silently overhead at 30k feet. Sonic booms are dramatically less stealthy.
If it had a very small radar cross section then it could be an interesting cruise missile, and if it had a robust pressure hull it could be a submarine. It doesn't. This conversation is stupid.
