With regards to St Athan about the only thing going for it is that it has a railway line alongside it and past the end of the runway, against it are that it is even further removed from Cardiff than is, the former, Glamorgan/Rhoose airfield and it only has a 6,000ft runway with public roads and a railway line off the runway end(s).
However ... the downside to that particular railway line is that it is the back line from Barry to Bridgend, the same line that serves Rhoose village and the current airport from there. There simply isn't a catchment for more than one train an hour.
Yes, there are bus services, by two operators, both to the current airport and past St Athan airfield to/from Llantwit Major but, again, services are hit and miss and come early morning and/or late evening "if at all".
This is what comes of utilising former military airfields out by the coast and/or in the countryside, they are rural locations with associated rural public transport services.