I would tend to think that a DRL would end streetcar service on King. A DRL route would parallel King for much the the route, and given the choice between the streetcar in mixed traffic or walking a few blocks south to catch a subway / train in it's own ROW, I wouldn't imagine that many people would bother with the streetcar anymore. The 504 would stop completely, I'd imagine. The 503 Kingston Road route could drop people at a Queen stop on the DRL east of downtown instead of coming right into downtown. The 508 Lakeshore could drop people at a King stop (around Liberty Village, depending on the routing), or could just stop altogether and those people could take the Queen car.
Not sure what would happen to Spadina / Queen's Quay (510) or the Harbourfront (509). It parallels the route for a small portion, but those routes provide service North and West that the DRL wouldn't, and the Queen's Quay service extended east to the Portlands would provide a reason to keep service for the full length along Queen's Quay.
I don't think a DRL would eliminate any of the other routes further north.
Greg