Reading the comments on Munro, I am now thinking that the best option is an extension of the subway to Steeles combined with LRT north, all the the way to Elgin Mills. If it is demonstrated that the maximum passenger carrying capacity of the LRT line will be enough to meet demand for some time to come it would provide a transit option for the greatest number of people in the shortest amount of time.
York Region takes greater control over the project and it's timeline, the fight between the disparate municipal governments working at cross purposes , each in the best interests of their citizenry, comes to an end and we don't have to wait until the DRL is built to begin providing transit to residents of York Region.
....
York Region is the fastest growing in the GTA and Markham, Vaughan and Richmond Hill all have solid growth plans that include urban density throughout the Yonge Street Corridor. This is a goal that should be supported but the bickering over which transit opportunity should have priority only serves to stagnate the disirable development.
Honestly, even if LRT could handle the capacity (and it can't), this is the exact problem we have now in Toronto. The province and the region already picked subway. Metrolinx is pro-subway. The EA is done. There is no reason for any more "bickering," though you wouldn't know, watching the Scarborough debate, that even starting construction means a deal is a deal.
If you KNOW it's the fast-growing part of the GTA (and top-3 in Canada), would you bring a subway right to their doorstep and stop it when all their planning is based around the capacity it unlocks? Spite? The whole problem is, as you kind of point out "municipal governments working at cross-purposes." That's why we have a REGIONAL transit agency to develop a REGIONAL transit network so you don't have one municipality trying to screw another and then that municipality going, "Oh, we really need a subway but I guess we'll just build an LRT because it's easier." (Or even worse, "We really need an LRT but we'll build a subway, subway, subway, because that's what the people DESERVE.")
Why would you stop and start again to look at an LRT? Make a plan, fund it, build it. The inability to pull this off (along with a lack of funding, of course) is why we keep falling further and further behind. Stopping the subway at Steeles would be just like building the Sheppard line to Don Mills. Keep nickle-and-diming your transit, keep doing the easy thing instead of what's necessary to get out in front of the problem, and keep watching traffic rise and sprawl increase, says I.
I'm happy some people corrected the comment above that the density is NYCC or below. It's higher in Markham's section, about the same (give or take) in Richmond Hill's section and that doesn't even include the development along Yonge Street itself. So, comparable to NYCC at worst and ideally higher. And NYCC wasn't starting from scratch (i.e. it was on Yonge Street, the civic centre was there, there was 2-story retail etc.) whereas these communities are.
The provincial minimum target is 400 people/jobs per hectare. RH's plan is for 450 and Markham's (pictured above) is for 1,000. Give them an LRT instead of a subway and both those numbers drop, especially the latter.
Oh, and Vaughan has raised densities along Yonge but it doesn't have anything at the terminus because there is a mature neighbourhood that backs into the hydro corridor. It's over 100,000 new residents by 2031 (though the subway delays likely put that date out the window).
EDIT: I'm adding a good graphic I found of the entire growth centre; both the Markham and RH plans.