If you're an established player, keeping essential assets out of the hands of new competition has value in itself. Example: when the Vons and Kmart from my home

If you're an established player, keeping essential assets out of the hands of new competition has value in itself. Example: when the Vons and Kmart from my hometown got together and opened a new shopping complex together, they both retained their prior leases for years, knowing that those locations were the only real estate in town large enough to house a real competitor. The fact that neither could utilize that real estate didn't make it worthless, and in this case actually made it more valuable to them: the town couldn't support multiple large retailers simultaneously, so any competition ran the risk of putting them out of business. The same concept easily applies to airline slots, an established player that is at capacity on how many slots they can profitably use still has some incentive to keep other airlines out, because any new players will be tapping into their existing market.