For the record my personal opinion is that a twice daily A319 to LHR with connections would be sustainable. I don't have figures for how many people from Cornwall fly from LHR yearly that could use a lhr link but with respect neither do you.
Cornishsimon, sadly nowadays it is not about whether a route is sustainable or not, it's about airlines looking for markets that generate the greatest returns and offer value for shareholder investment...
I get the whole debate about regional airports, in Ireland the debate has been huge especially since PSO's have been removed in the main. Why should local tax payers pay for a council investment that is for 160 people each way per day. BE know the stats, the revenue, the amount of P2P, transfer pax etc. if it stacks up, they would look at an alternate service to another London airport, I.e. LTN. If not it's says a lot to other potential airlines as there are no other operators. Given there are no other operators you'd imagine they would....