If Rouge and Air Transat disappear, so will many European connections to Canada.
With about 10% fewer seats than Rouge's 767s, I would have thought Mainlines 787-8s would have been a good fit for Manchester? Given Manchester to Toronto saw just under 150,000 pax in 2019, a daily 788 would have a 80% LF if all passengers stayed.
Failing Air Canada, might Westjet be interested in the MAN - YYZ market? Didn't they have plans to launch some flights from MAN to Halifax?
Things appear to have gone a little quiet at the moment with regards to the merger. Whilst shareholders have agreed the merger it would appear, according to Western Aviation News on 30th March, that it is now down to the Canadian government to formalize the proposal:
Transport Minister Marc Garneau has given himself until May 2 to conduct what’s called a public interest review of the Air Canada-Air Transat merger, though the pandemic may force him to extend that deadline.
You are correct as far as Westjet is concerned. They had planned to start a Manchester to Halifax service for the summer 2020 season but this has been put on hold. Whether they would be interested in taking on the Toronto and Vancouver routes would certainly be a good question.