Come back to me when the rail corridors stop running through empty industrial areas and actually go where people want to go.
Many of them do, though. The Georgetown corridor would conceivably provide good service to North-Western Toronto. More over, and this point is really important; the majority of any line's ridership will transfer from park-n-ride lots or surface transit, not walk-in ridership.
While that may not mean you can just put a line anywhere and expect it to be popular, there is a bit more nuance in terms of exact station location and considerations like potential surface transfers are more important than directly serving a modest cluster of density.
The bottom line is that there is no magical cheap transit solution to Torontos problems. People seem to love silver bullets, whether it be St. John hoping for that new refinery to come to town that will fix its employment problems (it won't) forever, or toronto where people are convinced that we can either build subways for free with magic fairy dust (or, ermmm, private money), or we can put a speck of money on our rail corridors and suddenly have magic trains running across the GTA travelling exactly where people want them at 5 minute frequencies and ignoring CN and CP who largely own and use those corridors. Here's a hint: it won't work. There is no such thing as a silver bullet, and the solution to torontos problems isn't a billion here or there on some rail line that runs to nowhere, but a mixed solution using the rail lines to move people between specific hubs and spending a substantial (and required) amount to build subways BRTs and LRTs to get people around locally. There is no single fix, the solution is a mix of small fixes that add up to a giant patch. As soon as people realize this, the faster we can move on and stop infighting over why the hell we aren't building the magic bullet solution instead of the stuff that actually works.
I don't think it's being presented as a 'magic bullet' or a situation where sprinkling pennies on the rail corridors will turn us into Paris. Most advocates of greater rail corridor utilization have been pretty clear that it would involve substantial costs. The most recent one was calling for 10s of billions of dollars as I recall.
And yes, RER-ifying a corridor like Richmond Hill is a bit ridiculous given it's basic course.
Nonetheless, if you compare Toronto with European or Japanese cities, the most obvious difference in our transit networks is the lack of frequent commuter rail service. We all look at London or Berlin and think 'wow, what a cool subway network!' while ignoring the FAR larger train networks which bring commuters into and out of those cities every day.
LRTs and BRTs and whatever are great and what not, but at the end of the day they're both marketing terms to make local transit more efficient. That's not meant to be a slight since marginal improvements to the local transit network will probably yield great results. There are limits to what steps like all door boarding, transit priority and prepaid fares can achieve though. And that's really what those terms mean in Toronto.
More radial lines are essential and would improve the operation of surface transit as well. Yet tunnelling a second "U" from Rexdale to Malvern via downtown would be so obviously cost-prohibitive it's a nonstarter. It wouldn't be free, but using existing corridors would be far more affordable.