No, it's not. The cost factor is minor.
For better or for worse, the perception of GO for a lot of people is that it is meant primarily to get people downtown at rush hours. When they then look at taking GO outside of those peak times, they get told that if they miss their train (or bus), that they will have an hour wait. And in that hour, they can easily drive (most of the way) to their destination.
Until such time that the trains run so frequently that people can just turn up at a station and not have to wait an unbearable amount of time - something that sort of exists at rush hour but not any other times - GO will be unable to shake that perception.
Dan
I think this will really depend on one's trip type; and origin-destination.
I live relatively close to Main Station and spend a fair bit of time downtown.
My travel time from Union to Danforth (Main) on GO is substantially faster (as little as 1/4 of TTC travel time and unquestionably less than 1/2 at all times).
However, if I'm traveling with packages/parcels/groceries, or its been a long day or the weather inclement, I'm likely to want a bus from Main Stn for my final leg.
The accretive cost (an extra $3.30) is material. I mean sure, I can afford it, but it feels foolish to spend that for an extra few blocks at end of my trip, when if I took TTC the entire way that bus is free.
So more often than not, I take TTC back from downtown; unless I'm unencumbered, and the weather is nice, in which case I take GO alone and walk from Danforth Stn.
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I imagine there are many similar'ish profiles; these would particularly apply to off-peak, discretionary trips, where one can plan to be back at Union at 'x' time and the missed train scenario is unlikely.