Hmm, the access route from Union Station to the new Union Station bus terminal seems even less convenient than the old one.
Here's the map that Metrolinx provides.
The Bay East teamway is directly accessible from the ground floor of the terminal. But it only provides access tracks 4-15 and 18-21, so people heading to other tracks will need to use the upstairs passage to get over to the Bay or York concourses.
I guess the main saving grace is that the amount of GO bus service to Union will continue to dwindle as off-peak train service is expanded. Many bus trips have been eliminated by the off-peak service on the Kitchener, Barrie and Stouffville lines, and I expect that the 16 Hamilton Express will be replaced by off-peak express train service relatively soon. Which will leave the 21 Milton as the only major bus route serving the 14-bay terminal.
I'm assuming that the plan is to close the Toronto Coach Terminal and relocate Greyhound, Coach Canada and Ontario Northland services to Union. For that purpose, I think the level of connectivity is more than adequate, it would be a huge improvement over the current coach terminal.