It would be nice, and as time progresses essential; but I'm challenging the statement that we don't need Spadina or Yonge or any of Transit City; but we do need DRL. Yet without any of those others, the capacity increases in the next decade should double the capacity of the line.
I've crunched the numbers. I'm including an excerpt from an article I wrote for the Ontario Planning Journal (although this excerpt is from an earlier draft of it, which did not make the final version, I had to condense it due to space constraints):
"Toronto’s rapid transit network servicing downtown is at or near capacity. In 2006 the Yonge Subway south of Bloor was near capacity (carrying an average of 28,000 peak hour passengers with a capacity of 30,800 peak hour passengers) (Toronto Transit Commission, 2009, p.5). In relation to the analogy, the water is barely able to flow freely from the funnel through the spout.
By 2031, the Yonge Subway south of Bloor is expected to carry an estimated 39,000 peak hour passengers (TTC estimate), or 42,000 (Metrolinx estimate) while having an increased capacity of 48,048 peak hour passengers, assuming that signal improvements are made, the new Toronto Rocket Cars are put into service, and an extra 7th car is added to all trains (Toronto Transit Commission (2), 2008, p.21-24). Even if all of these proposed measures for increasing capacity come into effect, the volume to capacity ratio will not be significantly changed, dropping from 90.9 percent in the present to 87.4 percent by 2031."
However, I am unsure if those estimates include the increased number of people who will be dumped onto the Yonge line as a result of E-W Transit City routes. I used the Metrolinx estimates for PHP for the percentage calculations.
EDIT: The final version is available on my website. I'd also like to add that I chose the title of the article before I did extensive research on the merits of various alignments, and I was simply going with the alignment that Metrolinx had proposed. I no longer endorse a Queen alignment for the DRL (although the article focuses mainly on the WHY we need the DRL, not WHERE we need the DRL).