1. Manpads are already pretty common
2. A-10 has both jamming and flare countermeasures against infrared.
3. Anything terrorist groups can acquire are probably going to be infrared MANPADs. My guess is ISIL would probably have acquired MANPADs from the large cache of SA-7 MANPADs Gaddafi stocked, meaning they're all likely infrared MANPADs.
A-10s are not invulnerable against MANPADs, but they're not helpless against them either. In fact, one of the biggest reasons the air force hated the A-10 was because it was "too slow" and could be "easily" hit by a SA-7
The A-10 is too slow. It is just used in a similar way to other planes these days, to drop smart bombs, a supersonic jet can cover a lot bigger area to respond to requests from ground troops.
A-10s are not invulnerable against MANPADs, but they're not helpless against them either. In fact, one of the biggest reasons the air force hated the A-10 was because it was "too slow" and could be "easily" hit by a SA-7