The original Key West agreement of 1948 identified CAS as an Air Force responsibility.
https://en.wikipedia.org/wiki/Key_West_Agreement
The Army totally got out of the fixed wing business in 1967, while rotary wing CAS and logistics became an Army function.
https://en.wikipedia.org/wiki/Johnson-McConnell_agreement_of_1966
The bottom line is that fixed wing can do a lot that rotary cannot, and it’s the Air Force’s job. The A-10 is pretty damn good at it, and pushback from the Army should be expected.
All well and good, but the times they are a changin’.
In the future, CAS will be transferred to drones at the regiment, squad, or even individual soldier/Marine level. Controlled by individuals on the ground, in real time, by means of eyeball tracking or something even more exotic.
Eyes in the sky will communicate with individual warfighters, sending them constantly updated views of what’s around them. Look at a building, look through the building to what’s on the other side, look into the building to see what’s there.
Army/Marine responsibility.
It’s 2020, so let’s revise the 1948 agreement. The world changes, just look at Space Force.