You will have GO RER up till Kitchener, and London via SML, for people who don't want to spend that much, and don't mind spending more time on the train. And you'll have HSR for people who want to save time. So the only people left to service on that line are riders getting on/off at Stratford and Saint Marys with VIA. I don't see the business case to continue service here. Hand off to Metrolinx and give anybody from a VIA train, due for a destination on NML, a GO RER or HSR transfer and run bus services to Saint Marys and Stratford as needed.
VIA could, if they have the capacity for it, split their HFR service at Union with half the trains on the NML and half on the SML. But again, you'd need a solid business case to do that, vs just handing those passengers transfers onto the HSR and RER. Say you're coming from Ottawa or Montreal and travelling to London. Why would you want to stay on the train for a longer trip, vs. just getting a transfer to HSR (included in your VIA fare)? Sort of like how GO and VIA offer combined fares today.
Handing off also means that VIA could concentrate services better. Maybe HFR becomes an hourly through service (at Union) till London on the SML, with the NML left to Metrolinx.