Which is my main issue with LRT and TC specifically. Building Eglinton as an LRT and stopping Sheppard before its logical terminus at STC limits the TTC's ability to interline and have a workable network so that in case of accidents or failures, there are ample tunnels to redirect the trains so that the network doesn't collapse; ...
You're not going to run Yonge trains via Kennedy to get around an issue at Lawrence. Nobody would sit through that diversion.
Neither Yonge or Bloor has spare capacity for this type of thing anyway. You would need to reduce Bloor frequencies by nearly 100% to accomplish a full Yonge line diversion.
Same problem for Eglingon. You cannot divert Eglinton south on Spadina/Yonge via Union without eliminating 50% or more of the trains north of Eglinton on Spadina and Yonge. The capacity doesn't exist. FYI, this is the same reason Eglinton will never carry more than 5000pphpd; there is no space on the lines they would transfer to.
Work cars are a bit different; but a good chunk of those have been built from old LRT vehicles before. Expect this to continue for Eglinton.
... also it introduces a whole new rolling stock, with it, new maintenance costs, building facilities, sheds.. It simply doesn't make much sense.
Not really. Rolling stock for Transit City is pretty damn similar to the vehicles ordered for the legacy streetcar network. I assume you don't believe we will have eliminated King, Queen, Spadina, Bathurst, College, and Dundas routes anytime soon. Even a full DRL doesn't predict a full elimination of the legacy network. The skytrain stock being eliminated still reduces us to 3 types on rails (Toronto Rocket/T1 and street-capable rail of two different widths).
The SRT, which is radically different and elimination of the H4/5/6 due to Toronto Rocket purchases leaves Toronto with fewer types in 2015 than we had in 2005.
Wilson isn't large enough, nor efficiently located, to cover all of transit city built as subway. See various yards discussions/documents for the Yonge line extension which resulted in a very large tail track.
Sheppard, Eglinton, and SRT as Toronto Rockets would almost certainly require either a massive expansion of Wilson yard (nearly doubling capabilities) or a separate set of facilities (preferred for redundancy, I.e. a major fire at Wilson yards).