500 is only a quarter full, but in general I agree with respect to Lakeshore West; however, the Lakeshore East doesn't have a greater population than any of the other lines. Residents of Durham Region, the second smallest region in the GTA, have no birth-right to half-hour GO service, and I doubt these trains would even be a quarter full at mid-day or on the weekend. Lines like Stouffville and Milton, that have nothing but jam packed rush hour trains and mostly full off-peak buses, would be the ones subsidizing that service.
I think the weekend Barrie train service just needed more time to catch on. It takes a while for people to figure out that the service exists and that it's a good alternative to driving. The Lakeshore lines have had decades to build service!