I understand the argument that ridership might not be strong enough to justify a delay for Union-bound riders, but I don't understand the objection about cost. The simpler of the two ideas I suggested is just a side platform south of Bloor, next to a track which Metrolinx already plans to lay. It's just a simple concrete pour. There wouldn't even need to be elevators, just a ramp down to Bloor St. I think that would be pretty cheap, though you might need to expropriate a tiny sliver of land from the school board.
If you did it right, it could also be really good for the local neighbourhood.
I didn't realize this earlier, but the Loblaws south of Bloor is about to be redeveloped. In the project thread,
someone was complaining just this week about the 500 metre walk to the GO platforms. With a Milton side platform, there could be a platform at the front door. Plus, a platform there might make it easier to talk the Sterling developers across the tracks into paying for a pedestrian bridge. Both developments would help with ridership too. You could even expropriate and demolish that industrial building at 64 Golden Ave, which could open up the commercial buildings south of Golden to re-development as well.