Lets try to avoid this hyperbole and stick to the numbers.
Sheppard has 4,000 riders.
The right of way LRT can comfortably accommodate 10,000 riders.
The underground LRT can comfortably accommodate 20,000 riders.
So it's reasonable to say that heavy rail subways (Toronto Rocket cars) would be warranted when Sheppard approaches 20,000 riders.
So what you're trying to convince us is that ridership on the Sheppard corridor, from Yonge to Scarborough Centre) will increase five times in the next few decades, enormously outpacing the growth we see anywhere in the GTHA.
That won't be happening anytime soon. I can guarantee it.
Your numbers are a bit higher than what I would have used, but close enough (yours seem closer to peak and not "confortable").
The lower threshold for ridership should be about half the confortable level. Thus, the threshold for subway is closer to 9 or 10k. Using your number, the DRL is not needed (IIRC, the last study had the ridership at about 17k).
Also of interest is the fact that neither the Sheppard or Finch LRT are actually needed. I think their forecast is about 3200 and 2800 respectively (in 2031?). Buses (artics) can handle 3600 ppdph (60 passengers x 60 buses per hour)
Does Sheppard need a subway now - NO
Does Sheppard need an LRT now - NO.
So I suggest using busses in the interim, cancel both those LRT's and build the DRL now. In 10 or 15 years we can revisit the Sheppard situation again, but lets leave the options open for subway or LRT - thus, do not loose the key points needed for construction.
My Calcs:
Median LRT: 200 passengers x 2 car trains x 20 trains per hour = 8,000 ppdph.
Median LRT and higher frequency underground: 200 passengers x 2 car trains x 15 trains per hour + 200 passengers x 3 car trains x 15 trains per hour = 15,000 ppdph. (3 car trains run in underground portion, but frequency lowered to accomodate interlining.
Fully underground LRT: 200 passengers x 3 car trains x 30 trains per hour = 18,000 ppdph.
4 Car Subway: 150 passengers x 4 car trains x 30 trains per hour = 18,000 ppdph.
6 car Subway: 150 passengers x 6 car trains x 30 trains per hour = 27,000 ppdph.