I think there's a tension right now between what UPX is right now, and what it will be as GO improves on the Kitchener corridor.
IMO the right long term stops for the UPX are Pearson, Woodbine, Mt Dennis, Bloor, Union, East Harbour. Woodbine with an extended Line 6, plus as a transfer point from GO services west of Woodbine. Mt Dennis and Bloor for the Line 5 and 2 connections respectively. Union is obvious. East Harbour as a transfer point for the Ontario Line and those going to the airport from eastern Toronto.
But SCOW, Weston, Liberty Village, etc, should all still exist, just as GO train stops. With GO trains running every 15 mins or better it would meet or exceed current UPX service levels. I suspect we'd see demand for 10 minute or better service in short order.
IMO that provides a cleaner separation between the "airport express" and "RER" service patterns. But any actual implementation of this is obviously years away, and depends on electrification, new operations at Union, etc. So for now there's obvious demands on the UPX because the service frequencies as so much better than GO, and that creates this tension between two good goals (express airport service and good local transit), and we're going to see good arguments on both sides.