Except that the 35% (on average) of connecting passengers on each flight to/from Heathrow make many routes viable when they would otherwise not be.
I'd hardly call that "no value added".
Airline Economics 101.
And what about the distribution? Is it 35% +- 5% across the board, or 90% for some routes, 5% for others?
Bear in mind also that with the huge cost of a slot at LHR airlines need to schedule fewer larger planes to spread the cost. A high capacity transit hub may make it feasible to schedule smaller feeder aircraft serving more numerous terminal destinations. So instead of driving/taking the train to T.E. you might be able to catch a connecting flight via small jet/turboprop from your local airport in the UK/north of France/Belgium/Holland etc.
Econometrics 101 ;-)
Golf-Sierra