At the current rate, it would take over a half century to pay off the existing $6.8 billion in debt. And privatization isn't free. The whole goal is to sell the aiport so the government gets money to build new infrastructure. So add several billion more to that existing $6.8 billion. Possibly even a doubling of debt (article says $5-7 billion for the sale of TPIA). But they'd no longer be paying rent to the feds, so they'd have another $180 million annually at their disposal. But with $300 million annually to pay down $14 billion in debt and the interest of that extra $7 billion, there will be no fiscal room at all to improve the airport and do grand projects like the transit hub.
What if the government decides to sell the airport in order to build the transit hub? The transit hub would then be a public asset, with the airport itself being a private one. The ROI on the airport would likely be higher than on the transit hub anyway.