It's clear that a Scramjet MUST fly at supersonic speeds, normally above Mach 2 or it simply don't work.
Where does that come from?
An I-15 Biplane was used for testing of early ramjets and it succeeded in accelerating the aircraft to about 45km/h faster than the aircraft was able to fly with its normal engine alone... and I can guarantee that the I-15 is not supersonic.
(yes, I realise 45km/h faster is not much to write home about, but with the added weight and added drag of the two new engines and all the piping and extra fuel is is interesting in showing potential...)
Here is an image:
Ramjets and scramjets are more efficient at higher speeds but that does not mean they can't operate at lower speeds.
Zircon is intended to fly at high altitude, but could fly at lower altitudes and at lower speeds... just like the ramjet powered Moskit can fly at higher altitudes at mach 2.5, or at sea level at mach 1.8.
Very few things can fly faster than mach 2 at low altitudes... everything is effected by the thick warm air.
The Zircon flys at mach 7-8 because that is what makes it safe.
With airborne aircraft using radars a US carrier fleet wont be surprised by a sea skimming missile so the two solutions are stealth or fast... The Kh-101 and Kh-102 are stealthy and fly low and the Zircon and Onyx fly fast.. Zircon being three or four times faster than the Onyx it will replace.
Detecting an incoming Zircon at 1,000km means you can turn on all your radars and SAMs, but at mach 8 it will be approaching at 2.5km per second, which means you have 7 minutes before impact.
The simple fact is that in 7 minutes you wont have time to vector an aircraft to anywhere near an interception position unless they are already in the air and already in the right position... and even if they were what are they going to do to a missile passing 20km plus above them at 2.5km/s?
Missiles like sprint or Standard are designed to intercept targets at lower altitudes and ones that follow a relatively fixed ballistic path, so their chances are not great even if dozens are launched at the target...
Zircon will be a very potent weapon.