Why madness?
Train from GVA to Zurich 3 hrs
Train from Lausanne to Zurich 2:15
Swiss Fly a mix of CS100 and A321 to ZRH 9 times a day from GVA, and that will be a mix of long haul connection and city traffic.
They don't do it for the fun of it and I'm pretty sure that the local market of GVA is not the main driver of the capacity available.
Taking 150 of the 1500 available seats by air plus the capacity of 16 trains a day between the two cities is telling that the route is very very busy, add in that the Valais has a population of greater than 350,000 - ie on a par with Bristol and bigger than Cardiff, then add in the roads getting into GVA in the morning - think M25, and it may not be such a mad idea? I know from experience I'd rather turn right out of Lausanne or Montreux in the morning to take a flight than turn left and head to GVA.
However time will tell