I think there's a flawed premise frequently used in these transit-related threads, and that's to try and argue against LRTs by making subways into something they're not: commuter trains. The straw man invariably put forth is that streetcars can't travel quickly over 20+ km distances and therefore subways are required.
At the risk of stating the obvious, streetcars travel in mixed traffic while ROW LRTs do not. Stop spacing and the frequency of traffic lights are what ultimately determine how fast a ROW LRT can get.
Similarly if commuter trains are what's logically required by the basis of this premise, it's illogical to push for subways since, ownership issues aside, virtually all of the rail infrastructure is already in place and it would be far more cost effective to simply add commuter trains and enhance stations and transfer connections.
Realistically, a mix of ROW LRT, subways and commuter trains will probably be needed in the not too distant future. And bear in mind that the first and the last of these three will be by far the most cost effective to build.
Conceptually it is correct that commuter trains are better suited for long trips than subways. However, currently we don't know how much it will cost to upgrade the rail network to the point when it becomes a viable option for multiple trips patterns within the city.
At the Sheppard level, there is no E-W rail line for crosstown trips; hence I expect the Sheppard subway to become essential at some point in future (even though it should not be the #1 priority right now).
Quite a large number of E-W crosstown trips could be served by the Midtown line if it had train service on 15-min frequency or better. But the existing tracks are heavily used for the CPR freight traffic, and space to add more tracks may not be available for some sections of the corridor.
The inner suburb-to-downtown trips are in a better shape as there are multiple rail lines to Union. Still, a lot of issues have to be resolved before GO trains can play a role similar to that of S-Bahn in Berlin or RER in Paris:
1) Capacity at Union; they do expand it but not very quickly. There is a risk that all capacity growth will be eaten by the growing 905-to-downtown demand, with nothing left for 416.
2) Width / number of tracks in the corridors.
3) TTC route structure. Currently, getting to a nearby GO station by TTC often requires a transfer between two TTC routes, or a long walk from the "closest" TTC stop.
And then, of course, there is an issue of fare integration; but I'd think that it will be resolved if the above mentioned issues are resolved.
So, it is doable, but the costs might be considerably higher than they seem at the first glance.