Read my response again, as it's pretty clear my thoughts on all of that.
No one is seriously suggesting that interlining is an option. I certainly didn't in my previous reply, so I'm not sure why you're bringing it up.
Does it though? For as "off the shelf" as they bandy it all to be, the equipment used on the Ontario Line is no more off the shelf as the current Toronto subway stock. Hell, there is basically no "shelf" for subway equipment as it is all so highly customized for each location it's required to run in.
What isn't customized, however, is the equipment used on the rolling stock. Motors, gearboxes, air conditioners, control equipment, doors, couplers, brakes, to name but a few - those are all elements that have multiple suppliers and are all built largely the same. And it's no different for the existing subway cars.
I don't know why you think that only the Ontario Line could be made into some interregional line. The subway has run outside of the City of Toronto since 2017.
The way the Ontario Line is currently scheduled to be built, it will be just as awkward to use it as "medium capacity transit" as the current subway system. It's certainly no more flexible (and arguably less, as it can't be used on the existing subway).
The difference with the LRTs is that they aren't sharing any of the network with the streetcars, and there are no places where that could happen. And there are very few places where it could interface with the existing subway system. That's why I've never argued that building them in the way that they have was a mistake. That is the exact situation where building an independent system makes sense, although the fleet size (especially of Finch West) worries me.
None of that is the case with the Ontario Line though.
Dan