I think we need proper perspective here; but first, in order to do that, why don't we see exactly what Jenn had to say:
"to the relatively small ridership potential. But still, good."
I think 'relatively' is an important word here; and that implies a comparison.
GO's ridership in 2015 was ~13M and change, and according to this report:
The potential in ridership growth was 140% with fare integration etc.
So about another ~18M rides annually, based on the 15M service model.
That compares with TTC ridership as a whole at ~450M
So the growth is equal to roughly 3 weeks of ridership of Line 1.
In relatives term, the ridership potential is low.
Also, worth saying; Jenn finished by still endorsing the concept 'But, Still Good'.
I'm a proponent of GO RER; but the entire thing put together is fractional relative to the TTC unless you can drive capacity higher, by driving frequency higher than the 15M in the original plan.
That seems likely, I should add, but even at double the original plan (every 7'30) the capacity enhancement isn't huge relative to a single major subway line.
Now if you can get that time to 5'M or less.........