The figures need to be looked at carefully and interpreted. Just a few points.
The 2011 Census had the population of W Midlands Planning Region at 5.6 million not 5.3m.
Birmingham is also the most attractive airport for a chunk of surrounding territory outside WM such as mid-Wales, N Oxfordshire. By contrast even CF would not be clearly the best located airport for Sheffield,certainly not the affluent parts of Sheffield.
Looking at Bristol and E Mids as the best comparators to Leeds and looking at the CAA stats, I reckon an all singing all dancing relatively unconstrained airport would be doing a couple of million pppa more than Yeadon. Given that would never be a purely commercial proposition, how much public money is it worth spending to achieve that? A fraction of what it would cost IMHO.