I must say, I don't see where it says that 100% of vehicles at all times, must be accessible, no exceptions:
https://www.ontario.ca/laws/regulation/110191#BK35
And it's pretty clear that the existing streetcars are not being made totally illegal:
Transition, existing vehicles
40. (1) Conventional transportation service providers are not required to retrofit vehicles that are within their fleet as of July 1, 2011 in order to ensure that the vehicles meet the accessibility requirements of sections 53 to 62. O. Reg. 191/11, s. 40 (1).
At best, I imagine that the TTC has an Accessibility Plan to convert the streetcar lines to accessible vehicles, because that is following the
spirit of the law, and they're doing their best to make that happen in a timely manner. Frankly, my reading of the law itself seems to indicate that the existing streetcars are forever exempt, and that WheelTrans is an acceptable solution, at least for meeting the bare-minimum.
The TTC is working to improve things well beyond that bare minimum, and I really don't see where it says that they flat out cannot use the old streetcars in occasional service in perpetuity.