Wouldn't the Branmpton buses be on Highway 7, and then go to to Vauhgan Metropolis City Centre?
The lowest ridership on the extension will most likely be at Downsview Park. It will have a useful GO Train connection, and some weekend ridership due to Downsview Park, but it won't be much. The buses on Sheppard will most likely continue to directly service the renamed Downsview Station (Sheppard West), Downsview Park is not supposed to have much of a bus terminal, if any. Is the 407 East and West buses really going to terminate at 407 Station? It currently terminates at York; I'd have assumed it would have at least ran through to Pioneer Village. Hmm, maybe that is what they are thinking ... but I can't imagine that's going to save any of the current ridership any time! Though presumably they could start running 407 East/West as a combined route then, which might have it's uses.
Still, the 84 Sheppard West alone is 12 buses an hour at peak and 6 off-peak. That's more passengers than the 407 routes. How many will change at Downsview Park rather than going an extra mile Sheppard West? There won't be a terminal at Sheppard West, but there'll be a bus stop ... and I believe you can cross Sheppard from the north side on the GO platform. Personally if I'm on the bus going past a subway station, I'm getting off - particularly given the long road approach and all the stairs at Downsview (Sheppard West). Hard to say how it will all work until you've tried both though ...
Ideally though, they'd extend the Sheppard subway to Downsview ... and then under the runways one more stop to Keele/Sheppard (which perhaps they'd rename as Downsview Park ... lol ...