If Lakeshore has so much off-peak ridership that it still needs 10 car double-decker trains even after 15-minute service is restored, then we need to be pushing for service increases beyond just 15-minute local service. Once 15-minute off-peak service is restored, 10-car trains would provide more off-peak capacity than some of the busiest railways in Europe.
For example, the mainline between The Hague and Rotterdam (where I used to live) runs 10 trains per hour:
2 intercity trains per hour: 7 single-level coaches (2x 500 seats)
4 regional trains per hour: 6-car bilevel EMU (4x 571 seats)
4 local trains per hour: 6-unit single-level EMU (4x 322 seats)
Total: 4572 seats per hour
Once Lakeshore West returns to 15-minute off peak service, it will have:
4 local trains per hour: 10 bilevel coaches (4x 1400 seats)
Total: 5600 seats per hour
If off-peak ridership is really that high then we should be pushing for the immediate introduction of all-day express service to Hamilton, in addition to the 15-minute local service to Oakville (30 minutes to Aldershot)