Exactly. A huge part of these projects is proving to the public the value. So even a short, direct line is useful - as some will start to use it and then extending it becomes a simple "this thing we have is good, it should be good more."
But the short direct line might also not get built, if the projections show passenger volume will not be high enough to justify the costs.
Passenger rail has high fixed costs and low marginal costs. Even with high-speed rail, you generally want to maximize the number of passengers rather than speed. Making detours to nearby major cities often makes sense, while stopping at smaller cities the route already passes through might not.
A direct connection between Toronto and Montreal would serve one pair of major cities, while a Toronto – Ottawa – Montreal – Quebec City route would serve six. The longer route could be economically more viable, even if the costs are twice as high, as the number of potential passengers is much higher.
I know this is super late and nobody will likely see it, but the population of Quebec + Ottawa together is 18% of the population of Montreal + Toronto.
And the amount of track to connect the 4 cities together is double the simple Montreal-Toronto route -- on paper. In reality it's much larger, because most of the track along the Montreal-Toronto corridor is useable for HSR, but the proposed Ottawa-Toronto stretch is the one that needs a lot of new track.
There already are trains connecting to all of these cities, so HSR would still benefit people trying to get from, say, Toronto to Quebec City (the current rail service has a ~30 min stop in Montreal anyway, a transfer wouldn't add any real delay in that respect, and you'd cut down several hours of the journey with HSR service for the first leg). I'm simply saying that it would be great to just lay the damn track for HSR between the two largest Canadian cities, and deal with the smaller ones down the road.
At the high level, it made huge sense to create a Toronto - Ottawa - Montreal -QC route.
Up until a few months ago, the plan was to create a new link between Toronto/Detroit/Chicago and upgrade the links between Toronto/New York City and Montreal/New York City. In this previous world view in which we were all friends, getting as many larger Canadian cities as possible connected to this rail network was worth the cost.
