No reason will satisfy you, so this post might be pointless. It's impossible to argue with the "not at capacity" crowd (as if utilizing capacity is the purpose of a transit system), largely since they apply this "at capacity" prerequisite only to subway lines.
You can drop the rhetorical dribble, and try taking a crack at fact-based arguments. The idea that capacity doesn't need to be utilized in a transit system is a complete falsehood.
TC is based on bus lines that are at capacity, although Finch East is mysteriously excluded, which many have noticed (and blamed on the politics of Sheppard). While it didn't target every bus route with high ridership, it distributed the network across the city for its first round (our currently proposed 7 lines).
Many years ago when the TTC was looking at opportunities for new streetcar routes, they actually sabotaged that report by setting such a high threshold that it was impossible for any bus line to reach that threshold (3,000pphpd, roughy what the King car carries) and thereby guaranteeing that there would be no new streetcar lines at the time that report was written (thankfully much has changed since). Spadina was struggling with demand as a bus before the 510.
Utilizing capacity is the purpose
of the mode of the transit system, although not the purpose in and of itself of the system (but the condition must be met to some extent... after all, people don't like seeing their tax dollars wasted on buses operating empty). If the capacity isn't used, then the system loses far too much money. That's how the system works. Capacity management is a critical element that guides the system. To claim it's irrelevant is extremely ignorant.
Yonge became a subway because of capacity issues. Bloor-Danforth became a subway because of capacity issues. The reason the Spadina subway has low ridership is because it never had a foundation of higher-order transit (i.e. streetcars) that was needed to establish the ridership base for meeting the demand threshold of a subway. Sheppard suffers from the same.
Capacity is absolutely critical, you must have a reason for spending so much money to provide so much capacity. And there is no reason for the Sheppard to STC argument. Pretty lines on a map does not an argument make.
Running the DRL up to or beyond Sheppard Avenue would do quite a bit to relieve Yonge & Bloor - this is undeniable -
Wrong. The biggest single source of relief for Yonge & Bloor comes from diverted Bloor-Danforth ridership, by far. In future, ridership diverted from Eglinton will also be a source of some relief kick, but this is only after the impacts of the LRT start to manifest in the system, and even then, it will be somewhat limited, although still worthwhile. North of there, the trimmings become negligible, because the LRT network can be built out from the north side of the 401 to divide and funnel the ridership into the appropriate nodes to divide demand to meet capacity, without investing needless expense into a subway that would provide far more capacity than needed. Let LRT handle what LRT is suited to handle.
You act like current bus routes are fixed in stone, however that is far from the truth, and Sheppard is actually a text-book example; look at how many routes were jigged to feed Don Mills. While LRT tracks are set in stone, rather literally at that, the integrated network that makes LRTs so flexible means their routes are not set in stone, but are limited to the network's existing track layout. A Finch LRT doesn't have stay on Finch all the time, but has limited points where it can turn off Finch.
but merging it with Sheppard will accomplish nothing other than squandering more benefits than I have time to list here (one of them is the maximum relief of Yonge & Bloor). It'd only affect 3 stations, and almost all of these 3 stations' riders would be going out of their way if they used the DRL. It's kinda silly that you think extending Sheppard east to Downsview won't relieve Yonge yet you think people at Bayview are going to travel perhaps 10km out of their way (or people at Leslie, 8km). A one seat ride from Bessarion to downtown is pointless. It serves no purpose and solves no problem and is very bad planning. I can guarantee to you that Yonge & Bloor will see some relief from a westward Sheppard extension, far, far more than merging the DRL with Sheppard would do.
I'm not expecting many from Bayview to use it unless they're bound for the Portlands, but Leslie would be dividing line. That's still affecting a majority of the users of the Sheppard line since an overwhelming majority of the traffic is originating at Don Mills. No benefits are being squandered because it's not going to have any impact on relieving Yonge & Bloor. I don't know where you get this ridiculous idea that everybody riding on the Finch E. bus is bound for Yonge and Bloor and contributing a massive load to the crowding there. That doesn't make any sense whatsoever. What do you know about transit planning when that's what you try to pass off for logic? If you have this long list of reasons, and hopefully they're better than the imaginary releif on Bloor-Yonge from such a far-north point, please share, but get off your high horse.
Do you even know where the Bloor-Yonge load actually comes from?! Half of the ridership on Bloor-Danforth east of and including Sherbourne is coming from east of and including Main Street, and one quarter comes from Kennedy alone.
It's kinda silly that you think Downsview would do more than the DRL when Downsview still wouldn't provide a one-seat. Transfers take time, and between subways they tend to be crowded, too, so there is incentive for people to take the one-seat as it saves time, even when going out of one's way, and alleviates crowding at system choke-points. Yonge is going to be pretty crowded by Sheppard anyway after the RHC extension opens, and that's what makes the DRL tieing into Sheppard very beneficial. It offers people the opportunity as well, to transfer from Yonge to the DRL when coming in from the north, or even northwest, for a destination in the east, like Wynford, for example.
I have to agree with scarberiankhatru. The increases in ridership will happen faster when there is an overall efficient backbone that allows the riders to travel to different parts of Toronto fairly fast and efficiently (i.e. a backbone) - to which buses can feed ridership to -- for those that do not live right on the subway. The more efficient the overall network is, the more attractive it is to some of those that currently drive. The DRL north should be left open for further expansion north - and the Sheppard line should be left open for further expansion - west/east (if it is not a completely new line - it will likely be easier to get funding for a station or two - than selling a totally new line when trying to expand the coverage of the subway.
This philosophy that subway lines can be extended forever is a bit far-fetched. A line has to end somewhere. Arguably, the DRL shouldn't go north of Eglinton at all, but since we're stuck with Sheppard anyway, which will never be extended as a subway in either direction, its best purpose and best bang for the buck would be to funnel riders away from Yonge as the north end of the DRL. Future expansion of the LRT network could see lines on Finch East and Victoria Park pour riders in around Parkwoods Village area, for example. Warden in Markham (which York Region is targeting for LRT) could come down to Sheppard and then across to Consumers, as another example. It makes a good radial point for the northeast, which takes some pressure off Kennedy and leaves mostly the straight east for Kennedy to deal with (the SRT will of course still be a strong feeder, but I said mostly).