Your logic is overly simplistic; the Shilka's predecessor was 57mm.
Which was based on Nazi 55mm cannons the Soviets captured, particularly because it can be easily linked to a computerized fire control system
as-is (a feature of the Nazi 55mm autocannon systems in question). In a period before Bofors managed to shrink the VT fuse into a 40mm package (and, in the 20-aughts, that prox-fuse was shrunk to
30mm), the only way you can get AA kills with medium/small-bore cannon (i.e., anything between 20 to 75mm range) is literally hoping to god that your fuses are set correctly or literally fill the sky with lead.
The US had a 75mm VT-capable system in the Skyweeper, but it was considered too incapable for air defense duties because jets were becoming so fast that any aircraft could outpace the traverse.
And my point stands. It's no different than other long range precision weapon support, ground or air launched. Who cares if your SDB was delivered by a B-1B, F-35, A-10 or being strapped to a rocket fired out of a HIMARS? What difference does it make? All the issues, pros and cons of arranging gun, rocket or dumb bomb runs are out of the equation, and CAS as normally understood is relegated to cheaply suppressing unwashed terrorist militias and the like.
Because you're still playing with the old definition when the reality is that things
have changed. The 'Dreadnaught Effect' has changed the battlefield and means that you can't fly NOE and survive. Remember, those S-60s and ZSU-2-23s counted for a surprisingly large number of US air losses, and Soviet SAMs were surprisingly effective for their time, being the
other portion of those significant number of air losses.
That was with '60s-era tech and literally learning the ropes (and basically
inventing the book, so to speak), mind you.
Search radar is going to be heavily degraded if not destroyed by the time Ground Air support is called in. Even with prestine search raydars, flyign nape of the earth is far more effective than any sort of stealth coating.
Nope, it isn't, Doom. We've made NoE a death sentence in the
'70s (the
DIVAD program managed to pull that off with its radars, the FCS needed a lot more work though), so that is a no-go.
If anything, the period between Vietnam and the end of the Cold War was something akin to the Dreadnought era of the 19-teens, where scientific advancement started to cause aircraft to be obsolete the moment they got into production.