Looking forward from now to next decade surely it is now the job of those who source new routes to bring services like Air Transat back
It's the same story at NCL as at CWL, BHX, BFS, EXT, LBA which have all lost their Canada services in recent years. The expat links perhaps aren't quite as strong as links pass through future generations, but I think there is a more fundamental issue. Back in the day, you'd get on a BA LGW-NCL or LGW-MAN flight and find an Air Transat, Nationair, Odyssey, Worldways or Zoom crew positioning as part of a trip pattern to cover operations like NCL-YYZ. It's both inefficient use of crew and worse still, pretty difficult to achieve nowadays given the deterioration of UK domestic air services.
If this is to stand a chance of coming back, three or four airports (normally competitors) need to team up to put together a plan as to how Air Transat could use a 321LR all week and how the crew, maintenance and catering logistics could work with services to YYZ every day from different airports. If you are Air Transat, I don't doubt there are less complicated ways to make use of your aircraft and you don't need to go out looking for headaches like this. If a joined-up solution is provided by the airports working together, it might help to restore regional Toronto services. Otherwise, there isn't a hope.