The A-10 is very good at what it does.
However, it was only designed to do one thing - kill tanks and strike ground targets at low level. It was meant to hold off a Warsaw Pact invasion of then-West Germany.
I had a buddy who bent wrenches on the A-10 out at Davis-Monthan AFB back in the '80s. He told me a lot about the A-10's capabilities.
However, it has weaknesses. It is slow, it has no radar, very, very little air-to-air capability (except for an enemy A/C unfortunate enough to find itself in range of the Sidewinder AAM's it carries or its massive gun) and they are wearing out (B-52s are too, but they are not called upon to do the kind of turning-and-burning that A-10s do at low level).
My former ANG unit converted from F-16C/D's and a dedicated Air Defense mission to the A-10C a few years ago...and now they are planning to convert to Block 60 F-16's!
Way too many F-16's were retired in the Afghan/Iraq Wars because they were not seen as useful for "boots on the ground" support as the A-10...leaving us with a massive gap in our air defenses.
The F-35 is a money pit. There is very little it does (other than overhyped "stealth") that the Block 60 F-16 cannot do.