Kolkata - London sector has adequate passenger traffic to sustain a direct flight. Even flight officials accepted the same when the Kolkata - London flight was withdrawn in 2022. More than the passenger traffic, it's actually the cargo traffic that brings profit for these international flights and Kolkata may lag in that sector. Moreover, along with the CUU-LHR flight, AI also cancelled BLR-LHR flight in 2022 citing lack of fleets. Recently in August, they reintroduced the BLR flight and as per news, the CCU-LHR flight is likely to be reintroduced by dec.
Also...
more... i dont agree with the fact that the no of MNCs decides whether a international sector will be served or not. For instance, take the example of Amritsar. It hardly has any big MNCs but still has direct flights for London and Birmingham. Same logic can be applied for Kochi that too has a direct flight for London. At the end of the days, it's the demand that a route has which decides whether that route will be served or not.