I'm sorry, I'm not really sure what you're asking.
When I talk about limiting demand, I am talking about not having any Union-Pearson passengers standing as a result of Union-Bloor or Union-Weston demand. The goal would be that during rush hour, trains arrive/leave Union with about 225 out of 240 seats occupied. If there are fewer than 225 seats occupied (as currently), the fares need to be decreased. If there are more than 225 seats occupied, the fares need to be increased, predominantly for the Bloor and Weston commuters.
The purpose of UP is not to serve demand along the corridor. That job falls on GO. The fundamental reason there has been such a push for UP to cater to local demand is that GO is not providing enough service. It makes sense to allow some commuter usage on UP if there happens to be spare capacity, but that must take a back seat to the primary objective, which is accessing Pearson Airport. Any commuter demand beyond that amount should be accommodated through increased GO service, which a far more cost effective way of carrying passengers.