Every mode in this city/region has its own set of network effects. To suggest streetcars can truly relieve the subway would force them into a role they are not designed for. The only way to relieve a subway network when pinch points are created is to design a “bypass” or something to that effect that offers a similar service within the same network. It’s just like when we build highway bypasses and such; adding capacity elsewhere to relieve an existing system constraint. To suggest LRT can replace a subway would be like saying Highway 7 can be an effective bypass for the 401- it is not, and we built the 407 (and also soon the 413) to do this instead.
Of course there is some leeway in this thinking. Line 5 is functionally similar to a subway in parts and will offer a similar service to Line 2 in their respective central sections. Hell, the Ontario Line isn’t the same technology as the rest of the TTC Subway, and it will probably fit it’s role neatly. The problem comes when the scale a service is meant to operate in is warped, and people use certain modes for trips they probably shouldn’t be. You could only consider the Queen car a candidate for relief is if you gave it subway like priority and stop spacing, which would compromise its actual, optimal utility.
With philosophies that contradict this, it’s no wonder Transit City failed. Much of the network was an upgrade to the burgeoned bus network with little attention to the strained subway network that said routes will feed into. We need both, and TC wished to tackle upstream local problems when the existing & ensuing downstream (trunk) issues would make all the problems far worse. It seems Miller thinks that just building transit elsewhere would alter travel patterns instead and make this a non-issue. Let’s see how that plays out.