Too much competition on the route wasn't directed at MAN itself, I know there's only them direct flights, both on the old 757s. But if you get a 45 minute hop from MAN-LON you've got LHR which has a hugely overcrowded JFK route. You might disagree but the prices are really competitive most of the time especially in comparison to the prices TCX are giving now (I appreciate these could drop).
With prices most likely going to be lower, more choices of schedules (early morning, early afternoon, early evening flights), better equipment and better onboard amenities and services I think generally most people will continue doing what they're doing now. You're right in saying a lot of people may well do breaks with them but I really don't think that will allow it to be a popular and successful route still.
Also with reductions in crew being mentioned in a couple of threads then it's hardly going to make it a great onboard experience, in my opinion. Because of this and the hefty NYC hotel prices, people may do this route once a year and realise how poor the onboard service is, which I think it will be, and never use the route again. After all, who's gonna pay more for less? Which it seems like people would be doing based at today's price. The higher cabins on BA/VS/AA are a lot more ahead of the game in comparison to TCX both in relation to comfort and service (from experience). Even at the moment the loads aren't that good on the 2 existing direct routes. Most pax will still go via LHR. And that's not just a reflection on the aircraft type they use at MAN.
They only thing that I could perhaps see working is MIA if TCX market it right and utilise the flight for cruises etc. and the popular Orlando routes.