Maybe not significantly, but it will help. While the bulk of riders transferring at Bloor-Yonge are from the east, some do come from the west.
If anything, the U to Dundas West will significantly help the 504 and 505 more than Bloor-Yonge. Why can't it be a Reflief Line for those routes too?
Because there's a limited amount of money. The goal of the DRL isn't to help the 504 and 505 but to help the at capacity Yonge line. Even the short version from University to Pape won't do much according to the planners. It needs to go to Sheppard to get the maximum benefit. Putting billions going west is counter-productive to the main goal of relieving the Yonge line properly. Why is this so hard to understand?