'And I can't really see why this part of the world can't sustain multiple international flights with the correct advertising'.
Unfortunatley, I can. Cornwall is one of the most economically 'depressed' areas of the UK and will never reach population critical mass to sustain such services. Newquay seems to be in vogue at present; it saviour might be its accessiblity for the Plymouth population.
Is BA still flying to LGW and has it affected Air Southwest greatly?