By definition, an express interregional service cannot handle local transit demand. No transit mode can be fast enough for long-haul trips, and at the same time have frequent stops to address local demand. GO trains do not handle local demand at all, nonetherless there is enough long-range demand to justify their existence.
So, the question is really whether an express east-west rail service can attract enough demand to justify construction and operation of such service.
In my view, such service along Hwy 401 cannot be justified in the near future (next ~20 years or so), because of its high cost and because of the difficulty of getting the riders to / from the express stations. Indeed, most of the prospective riders would have to use local feeder service at both ends of their trip, unlike riders of the GO trains to Union who can just walk from Union to their offices.
However, such east-west express service might become feasible in the longer term, as the regional population grows and the feeder route network improves.
This is only half true. On their own, you are correct that express services cannot effectively cater to local demand. However, you are ignoring the fact that if the stops on the express line are properly serviced by efficient and frequent LRT and BRT lines, that local demand can be effectively catered to. Just because you aren't within walking distance of a GO station doesn't mean that you can't access it.
It's not about individual lines, it's about the network.