In an ideal world a train option would be brilliant. However, mid Wales, as you all know if very hilly so the route has to go outside of Wales into England and then back into Wales again. Depending on where you live in Wales it is probably easier to catch the train to Cardiff. If on the other hand you live in NW Wales and need to get to SE Wales then the easier option, and quickest, is by air.
The train service is awful (as is the bus) and God help you if you're driving through mid Wales behind a tractor etc.
I think the airlink should stay as it serves a purpose and I really don't think it makes much (if any) impact on the environment. That's my opinion, for what it's worth