I could see the justification for both one day (BRT/LRT to serve more local needs, GO trains for regional distance), but not right now. What we're talking about here is a rapid transit service along a new corridor, with unbuilt and unproven ridership. Hard to make a case for a passenger rail corridor, which would have a high capital cost due to property, grade separations and geometric requirements (grade and curvature), and a likely a non-optimal operating ratio since land use around the 407 isn't very dense. Furthermore, some work would need to be done to create optimal connections in some places. Just looking at current 407 GO bus service: Hamilton/West Harbour GO, Meadowvale/Lisgar GO, Bramalea GO, Richmond Hill Centre, Unionville GO. And then there are others that buses serve away from the 407: McMaster U, Oakville/Sheridan College, Scarborough, Pickering, UOIT, Oshawa. BRT provides flexibility to dart off the highway to a nearby destination. Hurontario LRT and TYSSE are notable exceptions in that they're ready to go, however.
There are other outstanding questions, too. What will service look like: is this just for GO buses, or other local buses? What/how many local access point will there be? How does this mesh with the freight bypass?