Because putting both subway and LRT on the same route makes absolutely no sense. You have to build either one or the other along the whole length of Sheppard. Dumb political Miller transit proposals justified by rigged studies claiming that almost 0 population growth will happen along any of the "Transit City" lines should never ever be given funding.
Yes it does since at some point the subway makes no senses going past X point due to poor ridership. It should never been built in the first place as numbers didn't support it in the first place. Since Toronto/TTC only know Buses and Subway, they couldn't see Streetcar was the way to go considering Streetcar built the city in the first place. Then this is a Mel thing.
Yes ridership will jump along an LRT/Streetcar line, but not the numbers subway supports saw or think would happen.
Since the politicians will not support the conversion of subway to LRT like it should, you kill it at Don Mills like today and take the LRT east as plan. If the conversion had happen, then the line could go West to Downsview, tiering both lines together and then go west to Weston Rd before going on to the airport.