This discussion is nutty. More than 30% of York Region is protected land in the Greenbelt or Oak Ridges Moraine and, even discounting that, there's still a fair amount of agricultural and other undeveloped lands that are totally separate from this subway discussion. The Richmond Hill density figure is a prime example. I don't think there's a MORAINE running through the middle of Toronto, is there? Cut off everything in RH north of Elgin Mills (after all, the subway isn't going up to Oak Ridges) and that density figure will spike significantly.
So, Markham is only 1/2 of Scarborough. Let's go back in time 40 years, pretend we're discussing extending the subway from Eglinton to Finch and see if we don't find the exact same argument about the density in Toronto vs. the density in North York. Without that extension (and North York Centre) the spine of condos stretching from the 401 to Finch wouldn't exist. I still don't see how their previous non-existence is a reason not to have built the subway. Quite the opposite, actually.
One might as well throw in the density of Etobicoke as proof the DRL isn't needed. No one disputes York Region is a "suburb" (in that it's further from downtown than Scarborough) but to suggest there's some radical difference, for example, between Willowdale and Thornhill is silly. The density along the Yonge corridor is surely comparable, based only on what's there today. The whole reason you GET auto-oriented areas like York Region is by failing to build the kind of transit that allows Scarborough to attain that higher density. We're really missing the whole cause/effect thing.
"Scarborough is in the city and should get the subway first," is a statement with no logic to it whatsoever and substantiates my earlier point that if the Toronto border were at Hwy. 7, this discussion wouldn't be happening. Maybe we should forget about antiquated, obsolete notions of what constitutes "the city" and what constitutes "the suburbs" based upon lines drawn by provincial bureaucrats on a map 40 years ago and build transit where it's needed and likely to achieve its ends.