LA-I don't accept your analogy, in that LBA and DSA are not in that same 'high street', they both have their own. Had proper provision been made for two airports, one to serve Leeds and one to serve Sheffield many years ago, we would not now see the dominance that MAN has over the north of the country. But we are as we are, and we must now make the best of it (and one airport at Church Fenton is most certainly not that-it would be an attempt at compromise doomed to failure).
Manchester has most certainly aided many people from east of the Pennines over the years in that it has sent them on their holidays. It was this comparative ease that was used as an excuse to oppose both applications to extend the runway at Leeds. Whether MAN has helped Leeds obtain international inward investment in meaningful amounts is a very different question, and I personally have seen little evidence to suggest that it has.