> In practice by the time a frontline S-400 knows that there is something there, supersonic jets at cruising altitude already have too much kinetic energy to intercept.

The Swedish air force could routinely (16% of the time) get a fighter jet based missile lock on a frigging SR-71 in the 80s! 40 years later, altitude and speed do not help as much as you think. And radars are networked.

https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird