Well, if it weren't for them being lemons there shouldn't have been a need to even offer the option order, had they been reliable they shouldn't've had second thoughts about running them until 2019 and possibly until now, at which point there would be 2 models ripe enough for replacement...
Who do you think offered the option order to whom?
The TTC contracted with Bombardier for X amount of cars delivered over Y time frame, with the option to receive an additional amount of cars if they were pleased with the product. It is then up to the TTC to decide whether they want it (and then up to the feds, to decide how generous they are with the purse strings). How satisfied they are with the current rolling stock has no bearing on the existence of an option offer whatsoever.
As a counter example, the order for 204 Flexity cars had an option for an additional 60. Due to a variety of circumstances, that option was not exercised, and now we are paying more per car for 4604-4663, than we would be if we had taken the option when it was made available
Well, that is definitely not a realistic scenario, given that the T1 replacements will come at least 20 years later than the TRs (as of now, the earliest they are expected to show up is 2030, albeit with a more tight delivery schedule of 3 years instead of 6). Perhaps, with a 20-year difference in age, there would also be more technological advances, rather than keeping them almost identical to the TR.
Read back to the very start of the conversation, and you will see why I mentioned TRs and T1s retiring together. At no point did I say it was realistic.
But weight is a continuous spectrum, and so there will always be borderline cases or even overlaps.
And aesthetics or passenger carrying capacity isn't?
A car that weighs 20k kg empty will always weigh 20k kg empty. That doesn't change. But passenger carrying capacity can change - if you couple 10 crosstown cars together, you will have greater carrying capacity than a current TTC subway train. If you coupled 100 of them together you'd have a higher carrying capacity than the GO train. How's that for "light" capacity rail? (Note: this does NOT mean I would ever expect to see 100 crosstown cars lashed up together, just pointing out how capacity is a very unstable number that can change based on number of cars coupled together, as well as other things like service frequencies).
Aesthetics also change. You are a purist who thinks trams should be distinct from subways, because reasons. There are purists who grew up on the PCCs and think that the only real type of tram is the single bodied, corner driver seat model that doesn't view these new slinkies as being real trams. Someone who wasn't alive at the turn of the millennium thinks those singled bodied cars are as ancient and outdated as if they had been operating in ancient Egypt.
Perceptions are fickle. Hard facts like weight are not. And a streetcar, being a nimble, quick, electric, high capacity transit solution has a lot more in common with the subway, another nimble, quick, electric, high capacity transit solution, than it does with a VIA or GO or CN train. So no, I do not see any compelling reason to call subway trains anything but light rail.