In fact the A-10 was originally designed to fight the Soviet Union in highly conflicted air space. That fact that it has not had to do so lately is a bit irrelevant. Yes while it is an older designed aircraft its avionics have been continuously updated just as say the B-52 (anyone seriously calling for them to be retired en masse?) or the F-15. If you do not like the A-10 that is fine but be honest and not drag it down to suit your argument. The F-35 is not a close support air craft and whoever in the Army that is calling for it to do so needs to be fired now. Likely just another seriously flawed Obama general that needs pushed out anyway.
The A-10 was NOT designed to survive in its intended battlefield of defending the Fulda Gap from hordes of Warsaw Pact armor. They were designed to kill tanks and be expendable, merely to slow the horde, and were never expected to survive the battle or to stop the horde. Their role was to buy time.
Low and slow, and using a gun is the opposite of survivable.
In the 21st century A-10s are even far less survivable than they were in the Fulda Gap defense scenario. They only proved useful against the jihadis in pickup truck technicals sporting nothing more powerful than a 7.72 light machine gun or RPG to shoot back with. Arm those same jihadis with Stingers and the A-10s cease to survive. Arm those same jihadis with modern Iranian-produced mobile SAMs and they would not last 5 minutes on the battlefield. The Iranians are already doing just that with their allies in Yemen, arming them with sophisticated missiles which they used a couple years ago to attack our destroyers in international waters.
The fact is the A-10 Warthog will always have a job. We went down the road of multi-mission capable aircraft before and that is why we have the different array of aircraft that we have.