I think you are massively overcomplicating matters re EMA. The airport has fantastic private transport accessibility but its population catchment, although not that small, is fairly finite and confined. In other words the exact location of the airport on the M1 could not be better to ideally serve its catchment area population- it literally gets the absolute best from its relatively small geographic catchment. The reason why its passenger growth has slowed in recent years is because it has come under attack from the South with BHX's greater efficiency due to runway extension [runway extensions do NOT just mean ability to handle bigger aircraft]. Greater efficiency = lower costs and these lower costs suck passengers out of the East Midlands to BHX. The other main reason is that it has come under attack from the North and DSA. Many hundreds of thousands of passengers from South Yorkshire have traditionally used EMA because of the ease and time of getting there. This number has obviously reduced as DSA seeks to establish itself. Would it not have been better for EI to have more flights from one of the airports rather than divide the flights between 2 airports?
The real question that should be asked is are MAG really that interested in taking DSA on? If EMA wanted to beat DSA they, on a level playing field, could. EMA has the bigger population catchment. In other words why are BE flying daily from DSA to CDG when they could so from the larger population East Midlands? Forget sustainability - It's all to do with the deals these days.