First, a quick history:
After 1947, the USAF believed that strategic airpower could decide wars, whether global or local. Destruction of enemy will and industrial capacity through conventional or nuclear means would result in victory. Control over nuclear weapons passed to the Air Force because of the connection of such weapons with the concept of strategic bombing.
Ground support also fell under the purview of the new Air Force. However, the Air Force did not take to this mission with the same enthusiasm it exhibited for strategic bombing. Ground support inherently involved collaboration with the Army and consequently subjection to Army aims. Autonomy and the glory of victory would go to the Army, rather than to the Air Force. Still, the Air Force ensured that it would have a role in ground support operations through the 1947 Key West Agreement, which mandated that fixed-wing aircraft would remain under Air Force, rather than Army, control.
The A-10 was designed as close air support for ground forces. It's slow, so it has better time on target. Being slow, it's more vulnerable to anti-aircraft interdiction, so the cockpit is titanium for better protection. It's not a fighter aircraft, so it's not considered "sexy" by the fighter mafia who've run the USAF for decades.
The USAF thinks it's time has come and gone, and wants to replace it with fast and sexy.
The ground forces love it, because it actually supports ground forces. It's incredible firepower is an awesome thing to behold from underneath and behind it. It also tends to arrive rather quietly, then its engines blast mightily as it flies past, doing major damage along its path. Yeah, we ground types do love this aircraft.
When the USAF first proposed retiring the A-10, a number of senior Army officials proposed transferring the entire fleet and all personnel to the Army, since its close air support mission is so important to the Army.
The USAF had major heart palpitations over the very idea that Army would dare to go against the Key West Agreement. So it's kept the A-10 in the inventory while it develops other faster, sexier aircraft that may or may not provide true close air support. Remember, CAS is not sexy to the Air Force.
One argument against the A-10 is that it's an old airframe and costs a bunch to maintain. True, but then how does the USAF logically counter that argument when the B-52 airframe is so much older and still remains in the inventory (one of my Zoomie friends claimed he flew the same airframe his dad had flown)? Answer: The B-52 is a strategic bomber, which is one of the USAF's main missions, and therefore is sexy.
Both service positions are right. The USAF doesn't want to fly something it thinks is too slow, and doesn't want to do CAS, anyway. The Army needs serious CAS, and has had to configure as many rotary wing aircraft as it can to do the job the USAF doesn't want to do.
The Key West Agreement is long outmoded. Only the USAF still cares about it, and only that part that applies to strategic bombing and fighters.
The A-10 or a very similar CAS airframe should belong to the services (USA and USMC) who need it, not to a service that wants the glory but not the responsibility.
I love my Zoomie buddies, but let's be real here, folks. Y'all really don't want to have to coordinate with us ground types, so let us protect ourselves with fixed or rotary wing CAS airframes. It won't take away any of the USAF's awesomeness in its preferred missions.