Alvin: I wonder if we're talking about the same Burnhamthorpe! Your points are: B. has an established ridership. Yes, as do other streets, but this can be served by the existing bus routes. Connections with existing routes are certainly present, especially at the Square One terminal, but this would remain the same or even be enhanced if a busway were to connect to the terminal.
I really don't see much "intensification potential" along Burnhamthorpe, which is now completely developed along both sides, other than the City Centre area. Where would new density go, along Burnhamthorpe? The existing low-density neighbourhoods aren't going to morph into something higher-density.
By contrast, the proposed route running roughly along 403 would serve several higher-density nodes. At Erin Mills Parkway you have the Erin Mills Town Centre mall, and Credit Valley Hospital ( large institution, and getting larger). It would serve the City Centre and would be reasonably close to development farther north along Hurontario. Such a route could also directly serve the Airport Corporate Centre, one of the largest concentrations of offices outside the Toronto downtown core (and still developing), and obviously the Airport itself.
I don't know how you would serve these nodes efficiently from a route running along Burnhamthorpe. You would have to use connecting routes, with the need to transfer. Transfers add time and inconvenience to the trip and discourage riders. There is also the practical problem of how you would get a connecting route from any point along Burnhamthorpe up to the Airport, for example, without using existing streets? As a further bonus you could build "park and ride" lots at a numbr of points adjacent to the route. I don't know where you could do that along most of Burnhamthorpe.
In summary, why place a BRT or any other type of higher-order transit along Burnhamthorpe? There was a reason that the City abandoned this poorly-conceived idea years ago.
what is needed is an entirely different planning paradigm
No argument from me there, but it's easier said than done in an already-developed area where the prevailing "paradigm" is unfortunately that of 30 years ago.
My point in referring to "weaning the burbs off the car" is that we need larger-scale transit projects crossing boundary lines, which hasn't much happened in the GTA to date. The Miss. busway makes sense, IMO, only in such a context. I acknowledge those who claim that a line only from one boundary of Miss. to the other would not make the most sense. Most people don't actually care that much about political boundaries, they care about how to get from point A (home) to point B (work) and back again.
Scarberian: You're probably right, but AFAIK this discussion isn't on the agenda at present. I suspect the cost would be higher, but might be jstified by a higher passenger capacity.