"Ok, we're offering a $300 voucher to the airline willing to give up this slot... $300... ok, United, we're bumping you, you can pick up your voucher at the information desk."
The big airlines already do that. It's why the smaller airlines are forced into the smaller, outlying airports.
The airlines bought them. They're property. Just like the car dealer can't take your car back if you're not using it.
> You can probably personally blame me, at least to some degree, for the fact that United pulled out. When I was at American Airlines, we were consciously trying to push United out of JFK. That was our goal.
Once you give up those slots at an airport like JFK, good luck getting them back.
If you can afford a 5 year lease on a slot you won't actively use, you're still stifling the competition. And since EU prohibits EU-governments to subsidy airlines, this paves the way for non-EU countries with deep pockets to eventually control the EU air travel market.
If you want a slot to go to its most valuable use sell it to the entity that values it most, whether permanently or by auctioning it yearly.
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.