The RHSG could have relocated to the land freed up by the closure of 12/30. They could have done some kind of deal with the airport. That's a huge area which would have allowed them to built a purpose built exhibition hall and outdoor space, with plenty of land, much closer to the railway and tram tracks and actually further away from noisy active runways and taxiways. A new road network could have been built too, allowing dedicated roads to lead to the new RHSG for major events via the road that leads down to EDI Cargo and the current roads to be used exclusively for airport traffic with a bit of upgrading. The redevelopment of the current RHSG site could have allowed for a dedicated exit road from Newbridge to the terminal too as well as new spaces for the hotels and various other businesses currently being built around the airport and expected to be built on the new land. That would have freed up space for the current road leading down to the airport to be upgraded for people coming from the city/Gogar.
Sadly though, you are correct, there will be yet more congestion when the new developments start springing up,