There isn’t going to be any capacity concerns at Union after the USEP project, GO Expansion improvements, and the Ontario Line.
You really do have to consider that if you’re running the Barrie Line every 15 minutes, you’re only going to be having trains departing Union every 30 minutes under a split service plan.
Union has the central business district, entertainment district, is the transfer point to Exhibition Place, Pearson, and national rail, and is connected to 2 arenas & concert venues. Midtown Toronto doesn’t have any of that.
I’m not saying that there isn’t people on the existing lines that would like to go to Midtown Toronto instead of Union, but not everything needs to be a 1 seat ride, especially when it sacrifices 50% of departures in and out of Union.