I agree - the Peel plan for MME was to do exactly what they did at Liverpool and Sheffield City (and what Stobart's want to do at Carlise) - develop the land for commercial purposes using the presence/development of the airport as leverage to get want they want out of the local councils in terms of planning (local politicians love having airports, so long as they don't have to pay).
If they can develop a successful airport as well, that's a bonus (so yes at Liverpool and no at Sheffield) but it isn't core to their plans it just gives them an advantage when dealing with planners ("you need to let us do this so we can pay for your airport"). What has happened to Peel is the property market has collapsed. As AMC says, if the airport closes what will they do with the land - is there a shortage of brownfield development sites in Teesside??