"You're the one who opposes a Sheppard East extension beyond Agincourt, so you're contradicting yourself if you suddenly place onus on north McCowan when you laugh off a Sheppard-Markham loop which encompasses a Sheppard-McCowan stop."
The Sheppard line should run to STC because McCowan will receive its own line
I've recently become enamoured with this alignment: extend the Danforth line from Kennedy along Eglinton to Brimley (the existing platform strongly supports this), then along Danforth to McCowan, then up McCowan to Progress, verging ever so slightly westward to connect with the bus terminal and mall. From there, it's straight up McCowan, into Markham, to Markville/Hwy7.
Benefits: Feeding buses will draw from an area with over half a million people, Lawrence East and other bus patrons will get to the subway quicker (thus reducing the need for your Lawson line), it will compete less with Stouffville GO improvements (vs an alignment closer to the GO tracks), it will service the busy McCowan corridor, which is anchored by several malls, tower clusters, surprisingly dense residential neighbourhoods, and opens up the entire Malvern area to easy bus connections to the subway. Instead of running the subway through Morningside Heights, I'd rather run the subway through an adjacent area with about 8 times as many people - peripheral areas should be served by buses, not subway trunk lines.
"The official TTC plan to extend YUS into Vaughan puts stops as close as Jane/Steeles and Keele/Finch."
If being only one concession over is good enough, again, why not run the line through the busier areas so that the sparse areas full of parkland and golf courses are on the feeding-bus periphery?
"I think it's redundant to run two lines in a already well served area when a natural path to Rexdale/Humber College exists in the Sheppard corridor."
The natural - and shorter - path exists along Finch. From Downsview, York U, Jane & Finch, Rexdale, Humber, Woodbine, and Pearson, can all be hit in one curved line. Your Sheppard West line serves all kinds of Humber valley greenspace and the Belfield industrial parks (but not the airport) and it does so with a longer line. I don't care that the Albion area is not the "real" Rexdale so long as it has a lot more people.
"The jog would be less than 500m north of Queen for the stop."
Since you plan a John station, the curves required to get up to Dundas and back down to hit Bathurst may not even be possible - that's a lot of slow, tight, screechy turns. Queen & Spadina is really busy, anyway...you're avoiding it just so you can include a mezzanine modelled after an imperial Chinese palace.
"The existing 416 GO stations should be properly utilized before DRL."
But not before two lines to Malvern and a line all the way to friggin Rouge Hill?