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).
I think that there's a serious flaw in this part of your thinking.
It seems that you are assuming that most people are heading downtown.
I would suggest that the numbers say otherwise. If that were actually the case, then people would not be able to get on southbound subways each morning from Sheppard, rather than from Eglinton or St Clair as was the case pre-COVID. Instead, there is very heavy ridership turnover at Sheppard, at York Mills, at Eglinton, at St. Clair and at Bloor each morning.
I agree with you that most customers living east of Leslie (maybe even east of Bayview) and south of about Sheppard will likely funnel into the Ontario Line if they are heading downtown. But for a lot (maybe even most?) of the people who are destined for other locations, the Ontario Line will do nothing for their trips. Those people will likely continue on the same route that they've been taking all this time to get to their destination.
Yes, downtown is a still a huge draw for people commuting. But it most certainly is not the only location that people are headed to.
Dan