If someone came along and said they'd operate daily flights to London, Dublin and Belfast I'm sure Peel would be delighted to let them, and off the back of that I'm sure they'd also then be delighted to let someone bring a planeload of 200 people at a couple of random times a week too.
But while I'm sure there'd be people willing to fly the 200 or so holiday makers at two random times per week (flyGlobespan was a sad case; if it hadn't been a victim of fraud by it's payment processor I'm sure it would still be around), who do we think might might be willing to operate the daily flights to London, Dublin and Belfast?
In particular, who would be willing to run the service to London? Darlington has 2 trains an hour to London from about 5am to almost 10pm, and a lot of them take less than 2h30m. Even with the most welcoming airport operator in the world I wouldn't like to try and compete against that with anything other than another train.
Also, how do the conspiracy theorists explain Peel's other airports? If Peel are a hydra like monster desperate to land grab at airports why is Liverpool ticking along nicely and why have they done the total exact opposite at Doncaster? They strike me as a group happy, in circumstances where it's possible, to both operate and develop airports.