(Big fat disclaimer: I don’t understand anything I’m about to say)

Wouldn’t the windows be broken _behind_ the plane? In my mental model, you’re flying below radar so anyone on the ground will know you’re there anyway. If you’re sub-sonic people hear/see the plane and you hope they don’t have time to do anything before you strike. If you’re supersonic they hear/see the plane, their windows break, and you hope they don’t have time to do anything before you strike.

What am I missing?

You would leave a long trail of broken windows and reports from the moment you went supersonic in their territory, charting your trajectory. Threat detection is a network, not a single point.

However, if you're moving fast enough and the target is close enough, your premise is valid. They would not have time to react. Cruise missiles tend to be used for those mission profiles.

Has it perhaps occured to any of you that flying at below radar altitude at supersonic or above speeds, would be dangerous and would have no benefit?

If the enemy doesn't know you're there, you don't need to go fast.

It's what these guys and gals are trained to do. Ya, it's dangerous.

They do have systems that help and/or automate parts though.

Going fast into and out of AO is the way you stay alive.

Conversely, if you're fast enough you don't care who knows.

Going fast gives them less time to figure out that you're there. That's not always the right call but it is a consideration.

if the enemy knows you’re there but doesn’t know exactly where then it’s a huge benefit to get in and out as quickly as possible. it’s also a huge advantage if the enemy is moving fast and you know where he is, but you don’t want them to know that you’re coming at them.

If you are below the radar you are still visible and detectable. If they have any reason to suspect you are coming they can send people outside to look, you are low enough to be easy to see. Give that person training with a gun and you can be shot down. (speed makes it harder, but it also means the pilot does in the crash before having time to realize anything is wrong)