That is one big jump to go from buses to a full subway,
and why do people support LRT in other places where buses are full, what is so special about Sheppard?
It's just the nature of the corridor. First of all, it has the ability to connect the Spadina line and the Downsview developments with NYCC. Second, the meat of the line is anchored by two rather major growth nodes, which will continue to see thousands of new people and new jobs. Then, the entire Sheppard corridor is quite a high density corridor, littered with apartment blocks, and also the Consumer's business area/thing and Agincourt. Even if Sheppard didn't have
all this, it could still be an important corridor just because it's a much more northern RT line than anything else we have or could have in the city. We'd want to have
something in the north for people to use, and the fact that Sheppard is anchored in multiple places by high density nodes makes it the perfect candidate.
The TC LRT should not be taken as a form of RT at all. It should be looked upon in the same light as a bus service, because that is the kind of service it provides. Sure, it may be a bit faster and offer higher capacity, but at the end of it all, it does the same job.
If you look at dozens of European and the several Asian cities that are working with LRT, almost all of them are using LRT as a supplementary service. They already have their RT backbone to transport people around, so when a bus gets to the point of overcrowding, they upgrade it to LRT to improve capacity and faster speeds to the thousands of people who use the route; they can be justified for this higher speed because there's a lot of them, and the extra capacity solves the issue of overcrowding. To a lesser degree, it can also be used to milk out the last possible riders on a line, or to prop up a line that's not reaching it's full potential.