It is too tender, too ill-armed, too fast, and its crews will never have the detailed practical training needed.
The worst thing for the A-10 was that the USAF owns it - and if she can't have it, no one else can either.
The army will try to fill the gap with attack helos, but that is imperfect as well.
The folks at JQPublic have a nice summary of the issue - and set this point spot on.
In the background lurks a longstanding Army-Air Force tension in the area of how best to coordinate and deliver CAS. The Air Force’s institutional preferences turn on theater flexibility and efficiency, which explains repeated past attempts to push the A-10 out of service. The Army sees value in tight coordination and surface tethering in this mission area, and has reacted to the Air Force’s approach to CAS by gradually building up its own capacity to execute CAS its own way. What remains to be seen is whether divestiture of the A-10 will trigger new growth of the Army air mission, and whether such growth will undercut the Air Force’s ongoing fight to vindicate its institutional independence.
General Welsh says the decision to retire the A-10 was not emotional, but the product of logic and analysis. Opponents of the decision question, not without merit, whether continuing the F-35 program in order to avoid the loss of sunk costs is not itself an emotion-driven decision rather than a rational one. Perhaps emotion is a red herring, and this is really about the Air Force avoiding a confrontation with Congress over other, more sacred cows. One thing is for certain: no matter which of these theories best explains the drawdown of the A-10, the decision won’t be a good one unless it is debated vigorously.
Given the combat record of the Warthog, the firefight should continue until a winner is self-evident, and no one should wish for the A-10 community to willingly lay down its passions and unemotionally surrender. Given the lives that have been saved by the weapon and those that hang in the future balance, an uncontested retirement would disservice not only the CAS mission, but any decision of this magnitude.