The Air Force tends to organize itself to fight the most difficult war scenarios it can imagine. In the 60s it was poised to fight global nuclear war. In the 80s it was positioned to halt the Soviet advance across the Fulda Gap. Today, we're most concerned with an Air-Sea battle against China. The problem is, though all those scenarios are extremely difficult, none of them are very likely. The most likely wars, like Vietnam, Afghanistan and Iraq 2.0 & 2.5, are the ones that tend to expose our shortcomings.
Maybe the A-10 wouldn't be very survivable against China or Russia. But it sure would be useful against every other enemy we're facing now and in the future.