^In past years, there was an annual or semi-annual premium-fare special event train to Stratford. I don't know if VIA sponsored it or just sold a charter to some promoter - but it ran, even had a cocktail reception at Toronto Union. So the premise has certainly been demonstrated. If I were a Councillor or civic booster in Stratford I would be all over ML to do more of this.
There are some logistical things to resolve however. The least of which are the Halton connectivity on CN, the need for passing sidings west of Georgetown, and the general need for track improvement.
Then there is the problem that it's 980m from the platform in Stratford to the central Downie-Erie-Ontario intersection where the best dining spots are. And then 1400m from there to the Festival Theatre, and another 1600m from there back to the rail station. Not unpleasant distances for a walk, but possibly not appealing to the older demographic who represent a significant slice of Stratford Theatre--goers. There would have to be an assured shuttle service between these points, with appropriate timings.
Being a regular Stratford patron, I suppose I might use the service if it were offered - maybe. Stretching a lunch+matinee outing into dinner and a late arrival in Toronto (with a first/last mile leg required) could be a dealbreaker. Parking at Bramalea or Malton stations, and having a choice of after show/before dinner and stay-for-dinner return options would be the most attractive - I would be happy to skip the drive to Stratford part of the outing. For evening shows, I would not be eager to arrive back very late after a slow train ride and have to get home from Union in the wee hours.
If one assumes that Kitchener-Stratford-London route will become the VIA/GO backbone route before long (with or without HFR, and certainly long before anyone can sell new right of way construction to Oxford County) - then certainly building excursion appeal to a Stratford outing is a no-brainer. The article is compelling in demonstrating just how much potential there is for that much travel on the route. But personally I would not single out Stratford - add in many events in Kitchener, St Jacobs, Guelph....the route simply needs good hourly service all day every day. The business case is there, intuitively at least. Somebody just needs to get that moving as a project, without wasting years chasing studies.