U2 had no ability to detect incoming missiles and its cruise speed was only Mach 0.5The question has never been about whether flying at 60,000 or 70,000 feet will make you invulnerable to attack, as that has already been proven false by the downing of U2s decades ago.
The issue is the impact on your missiles’ range and NEZ of having to go so high up. Add in even 1.5m super-cruise to the equation and your range/NEZ shrinks massively further. To compensate, you need more propellant mass, which translates to bigger missiles. Which becomes a potential big problem when internal weapons bay sizes and VLS cell dimensions puts a hard cap on how big your AAMs and SAMs can grow without needing to invest in whole new sets of planes and ships. And this is before you even consider the fact that the CHAD has massively bigger weapons bays and so can carry bigger missiles than all existing 5th gens. Not to mention all the additional energy it can impact on munitions with its higher ceiling and speed.
Stripping out all of the next gen aspects, the size and ceiling of the CHAD alone allows it to outstick all existing 5th gens to a comical degree.
In term of energy, hitting something at very high altitude means interception happen when missile kinetic energy is at minimum, for U2 that wasn't a problem since the target made no attempt to evade nor has much ability to evade, but with 6th gen sensors J-36 can detect incoming missiles at long range, so with supercruise and altitude it will have massive energy advantage relative to the missile, which is to say not only does altitude suppress incoming missile's envelope, when combined with 6th gen sensors and high speed altitude also makes it much easier, perhaps only requriing a small heading change to escape the missile's already reduced envelope.