TC could, of course, be built with larger radii, clearances, etc., and still use TTC gauge.
What benefit is there to having two gauges for surface level transport in one city? Why separate the TC lines from the "legacy" network? What benefit accrues from restricting the interchangeability of at least some equipment? Why make something more complex than it needs to be?
There is an established gauge, that works, and interferes with nothing why change? Because London & Paris have lines that have different equipment?