There's something like 70 London to New York flights a day so in theory yes from a passenger numbers PoV it could support the A380 once there's terminal infrastructure.

But one of the reasons BA run so many flights (10?) a day is so that customers have choice and flexibility - it's not unknown for people to book business or first class seats on multiple flights in the same day so they can fly earlier if their schedule allows

Unlikely. Those who really want to get to Stockholm will not get on the NYC to London plane if they can get a direct plane. Substitute ALL other cities in Europe for Stockholm and a fair number of people are not on that plane. Those who do want to get from NYC to London have different schedules, some want to take the 5am flight and get to London in the afternoon, some want to get to the airport at a normal time and still get to London at a good hour. Some want to leave right after the days work is done. Some want a last supper with their family before leaving... Smaller planes are customer friendly. Also smaller planes leaves more room for redundancy - if one plane is broke you have less people who need to be rerouted to some other planes making it more likely you can get everyone to London without a major delay.