1) If you don't count the billions it would take to build the thing and finance it. This is why it could never be done by the private sector. And government's these days are loathe to take on significant risk. What government wants to be in charge when a $30 billion HSR has price and schedule overruns?
2) If you ignore the need to placate communities along the route.
These are very valid points. Personally, I think that if we look at a slightly less intensive HSR system, it could bring down costs sufficiently to be feasible. Rather than building a separate network of HSR, we could build high speed lines to bypass the parts of the existing lines with the smallest curve radii. For example, instead of building a totally dedicated line from Toronto to Montreal, we could build one from Oshawa to Kingston and upgrade the tracks the rest of the way.
To placate communities we need to have a reasonable number of stations in medium-sized cities (Kingston, London, Brantford, K-W, Brockville, etc.) although not all trains need stop at them. It's not worth the cost of building a high-speed line around the cities when there would only be a few trains per day going non-stop between major cities.
As well, have they looked at using existing corridors that are nearly straight enough for HST?
For example, between Windsor and London, the CN line is pretty much dead straight. To run at 300km/h we would just need to move all freight to the CP line, grade-separate the line (build a bunch of overpasses) and increase the curve radius on 2 corners: one near Chatham and one near Stoney Point.
To connect Montreal and Quebec, we could build a high-speed railway between Drommondville and Laurier Station in the median of Autoroute 20. HSR needs a roughly 15m wide ROW, and the median of A-20 is 25m wide. There are a couple corners on the route that could not be taken at 300km/h which the HSR would need to bypass, but even so, building a few kilometres of totally new ROW is still cheaper than building hundreds of kilometres. The best part of a freeway median is that it's already grade separated, saving the massive expense of building tunnels and overpasses.
The most realistic project I think we should work on is to upgrade the Oakville Subdivision (Lakeshore West GO). It should be 4 tracks between Burlington and Union Exhibition, with the centre two being for high speed (200km/h) VIA, Amtrak and GO Express trains, while the outer tracks would be used by GO Local trains and freight. It should be electrified too.
GO plans to electrify and quad-track the line, but there is no mention of increasing the line speed, likely because 95mph is adequate for our existing rolling stock.