So, perhaps instead of making the flight, the airlines should be allowed to pay a tax equal to 80% of whatever it costs to actually operate the flight?
2. "whatever it costs to actually operate the flight" is... well, good luck negotiating that bill :)
NB: they are not flying empty because no one wants a seat! They would be able to sell some seats! Even at previously-prevailing market rates! And of course could fill the plane if they sold the seats at any price!
They are flying empty because they can't sell enough seats at a high enough price to justify the fixed cost of the number of cabin staff required if there are customers on the flight.
It's about cabin staff regs, not about literally no one wanting to fly.
So... that 20% does matter.
And in return, they are maintaining a complex system that has no central authority and is not easy to change. It doesn't sound ideal, but I also know that it's not something the single "they should just" solutions that outsiders propose will actually work in. Hopefully things get closer to normal soon and we won't have to worry about it.
If there's something Europe needs, it's more taxes and more regulation.
By switching to a tax for unused gates/ flight slots that is 80% of the cost of flying an empty if plane, the Airline saves money, the airport makes money, and the environment has fewer CO2 solutions. It is a win-win-win on all sides.
If you switched this over to a tax you'd also need government programs to deal with the unemployment, lack of pilots and other ramp-up issues when this situation ends etc.