I think the question here is effectively if it is worth converting the Richmond Hill line or Barrie Line into Line 1, or simply using their ROWs for cheap but long extensions. Only sending the necessary number of trains of course- maybe half-hourly frequency or what have you. Luckily, Line 1 seems set up to use the GO corridor from Langstaff on instead of tunneling, but the question might be how far. Maybe to the end of contiguous Richmond Hill at 19th Line- but after that your converting or adding larger sections of incompatible track…
This seems to be the guiding philosophy behind things like the Ontario Line’s gauge and long term plans to expand it very far- it’s likely easier and more resilient to use proprietary tech than send TTC gauge trains to say, Burlington or Oshawa. But that’s not particularly relevant, and I’m bringing a second contentious topic (OL gauge) into this one, lol. Anywho, like you praised the OL for doing, terminating a subway above ground into a rail corridor always makes things far easier to extend, which is what we see here. The limits of that are up for debate, but way beyond our scope right now.