There is a huge reason not to use high end aircraft and that's cost!
Why use a B-52 that costs more to operate in one year than it would cost to buy AND operate a low end platform to do the low end jobs?
So after a very short period of time the platform has paid for itself in operational cost savings. Then in the long run you have saved billions that can be spent in other area's.
The real cost is in actually using a low end COIN aircraft. These types of aircraft have to be based in theatre and within the province they are going to be used at because of their limitations in range and transit speed. This means all their logistical support (fuel, spares, munitions) needs to be shipped into the theatre and to the base. This exposes your logistics tail to interdiction and costs a huge amount to defend.
On the other hand a 'high end' system like the AC-130 and a strike fighter or bomber can be based at a secure facility in a nearby nation and transit into the theatre for CAS missions. This means all the logisitics support doesn't need to be brought into the operational theatre and is much cheaper to sustain. It also means the CAS effort can be concentrated in those areas of the theatre that need it rather than dispersed around the theatre.
This doesn't factor into COMBAT effectiveness, survivability and a range of other issues.
I would like to see a purpose designed COIN/CAS platform for contemporary conflicts but it would not resemble a Vietnam war era platform. Things have changed a lot in the technology solutions and the need since then. The most ideal proposal I've seen to date for the contemporary COIBN/CAS platform is the CalPoly Firefox, AIAA competition winner for an A/X RFP.
http://aerosim.calpoly.edu/files/Firefox/Firefox_AIAA_Final.pdf