An example here is British Airways' service from New Orleans to London. Obviously, some amount of people have always wanted to get from New Orleans to London, but previously they've been flying though Dallas/Atlanta/New York, etc. The 787 allows BA to offer a much better product to those customers, and the size/cost to operate a 787 is what makes the flight commercially viable. When the sky is full of airplanes operating flights like that, there's no one group of people in one place to fill up a jumbo.
When I was a kid and lived in Santa Cruz, we would always have to drive to SFO for international flights because SJC's runway was too short for some of the bigger jumbos. Now with 350s and 787s, its not an issue.
Slot utilization at busy airports is one of the better pro-jumbo arguments going around at the moment, though. There's just not enough market pressure to cause airlines to push passengers away from direct flights and towards less popular routing options right now.
I think perhaps using London as an example is overshadowing the point being made. If you aren't aggregating passengers at hubs which you then use larger planes for the common leg of the journey, you don't need as many larger planes.
