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
Well BA run 12 flights a day between London and NYC (9 from Heathrow, 1 each from City and Gatwick) and it seems to work for them.
They're catering to the people traveling business class who want flexibility and it seems to work for them - they run something like 86 biz class seats and only 140ish economy on some flights
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.
You may be underestimating the number of passengers flying direct between two megacities sharing a similar culture/language.
Here on the west coast there is a packed to the gills flight almost every hour from LA to Houston, and more than one airline servicing the route as well.
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