As a ground-pounding CAS aircraft, there is none better. That effectiveness comes at a very high price, because when you see the A-10 swooping down in the mud with impunity, it is because me or someone else in a fighter was sitting above it at 20K feet watching it's back. A Mig-29 could turn an A-10 into aluminum foil in about 30 seconds if left alone with it. So, instead of one aircraft performing the CAS mission, you have 2 or 3 in a high/low combination. That is expensive. Effective yes, but expensive.
So, the Air Force says: "We can do almost as good a job with a stand-off CAS aircraft role for the F-16, and we dont need escort for it either". One F-16 can carry 6 Maverick tank-busters or CBUs in addition to a couple of AIM-120s and a couple 'winders.
So, they have an argument against the A-10. Not a great one, but a cheap one, which Air Force brass loves to hear about, if it gets them a few more -16s in the inventory. The A-10 lives on as it should, but the Air Force made a case against it that some could consider plausible.