It's hard for me to imagine how or why a customers originating from east of Leslie would favour the Yonge Line over the Ontario Line to get Downtown. That's another 5 kilometres stuck in a mixed traffic bus. The Ontario Line will get them Downtown faster. And even if it didn't, I'm sure most of them would still pick the OL just out of passenger comfort.
The Yonge Line will remain very busy, but at peak hours it'll be reduced to serving customers from York Region and the narrow band of Toronto residents between the OL and Spadina Line (which, to be fair, is the densest part of the city).