Why isn't the Bay Station as busy as the Yonge-Bloor Station (Bloor specifically)? Because there is no transfer-free connection between Bay Station and the Bloor Station. There is an underground connection, but it is outside the paid zone.
Any DRL Station will need to have a transfer-free connection with the Yonge-University lines. Maybe with multiple egress and ingress points, not just above or next to the stations themselves. With stores, of course, within the paid zone.
But again, there's one key difference there: for most people, Yorkville isn't their final destination. That is just a forced transfer on the way to their destination. For most DRL riders, their final destination will be the CBD, or the direct surrounding area.
The only trip that's going to see a significant transfer volume is people using the DRL to connect to the services at Union (GO, VIA). Very few people heading westbound on the DRL are going to transfer at King to go northbound on Yonge, unless they're really really lazy. Think about it, if someone from the east end of the Bloor line (say from Victoria Park), wants to go to Yonge and Dundas, why would they transfer at Pape onto the DRL, just to transfer again at King to go north by 2 stops. Wouldn't it just be easier to stay on Bloor-Danforth and transfer at Bloor-Yonge? Of course it would.
This volume can be easily estimated today. Just watch during AM rush hour how many King Streetcar riders get off at Yonge, go down into the subway, and then head for the northbound platform (or heck, even the southbound platform). For northbound, I would venture to say maybe a couple riders per car? For the vast majority of the King car riders, the moment they stop off that streetcar, that IS their final time on a TTC vehicle for that trip.
As for current B-D riders, I would venture to say that only a handful of people are going to walk more than 1 stops worth north or south from the DRL. So if it's under King or Wellington, down to Bremner or up to Queen is about as far as most people will walk. Specifically northward, if it's along Wellington, anyone bound for a stop at Dundas northward will probably continue to transfer at Bloor-Yonge instead of using the DRL.
Of course, because I'm me, I need to point out that a decoupling scenario renders this whole "which street is the best cross street" debate moot, because the DRL East would become the University line, so if someone coming from Riverdale wants to get to Dundas, all they have to do is ride the same line up to St. Patrick. Even if they're going to Dundas and Yonge, that's still a shorter walk from St. Patrick than it is from pretty much every "traditional" DRL alignment except Queen.