What can this do that an A-10 cannot do?
From what I have read, an A-10 costs about $18.8 mil apiece and $6,000 an hour to operate. A Super Tucano costs $8-14 mil and about $500 an hour to operate.
The A-10 was designed primarily to destroy hardened vehicles. As such, it is a bit of overkill in a COIN environment where the enemy is either on foot, or in soft skinned vehicles.
I've always thought there was a role in Afghanistan for a propeller aircraft such as a modernized A-1 Skyraider, or even modernized WW-II era ground attack aircraft.
The A-10s main gun, the GAU-8 is particularly effective in the rocks of Afghanistan. A short burst has an area effect like a cluster munition.
The downside of the A-10 is that it is a relatively expensive in relation to the role. Even the high velocity GAU-8 is overkill against soft targets.
Similar cluster munitions effect from the A-10's gun is possible with a less expensive airframe.
A modern propeller pushed/pulled ground attach aircraft having 4 30mm cannons such as the M230 Chain Gun used on the Apache helicopter would deliver a high explosive cluster of ~40 rounds per second. Add two more 30mm and you deliver a ~60 round cluster matching the upper end of the GAU-8's variable firing rate.
A further advantage of multiple lower velocity cannon is that with the A-10, the single GAU-8 either works, or it doesn't. If the GAU-8 doesn't work, the mission may be over.
On a propeller aircraft with multiple cannon, if one cannon goes out, you can still continue with the mission.
A reciprocating engine driven propeller aircraft has a tremendous loiter time relative to a jet aircraft, or turboprop.
So, there is a real niche for a propeller driven aircraft in a COIN role but I don't believe the A-29 Tucano adequately fills the niche.