If you want to appease Mississauga, the extension to MCC is the way to do it. To appease Etobicoke, a subway to Sherway would do it. So to appease the West End, you need only the Bloor extension to MCC, no one in the West End cares about a subway to the airport.
Which is why extending the subway to Sherway opens the door for Mississauga to take it from there, and arrange a similar deal to what Vaughan and York Region have already done with Toronto. Let Toronto pay for the extension up to the border, and then have the neighbouring municipality take it from there.
All Mississauga needs to run with it is to Sherway, and all York Region/Vaughan need is to Steeles. Same applies for Port Union into Durham.