> airport frequently have cheap or even free long term parking
Airport parking in Europe is pretty expensive. It could quite possibly be more than the flights for all passengers combined. A week at Brandenburg is about €150 Euros and at Heathrow is roughly the same (and needs a shuttle bus to the terminal, or it is over £250 plus for the short stay).
That is, however, still likely cheaper than a train to the airport in the UK and substantially less likely to have a cancellation cause you to miss a flight.
That's within the range for major airports in thee US as well. Whether I drive in (rarely) or get a private car, it's not hard to spend as much on going to and from the airport as it is for the flight. There are more budget options but they're not great for me.
Airport parking in Europe is pretty expensive. It could quite possibly be more than the flights for all passengers combined. A week at Brandenburg is about €150 Euros and at Heathrow is roughly the same (and needs a shuttle bus to the terminal, or it is over £250 plus for the short stay).
That is, however, still likely cheaper than a train to the airport in the UK and substantially less likely to have a cancellation cause you to miss a flight.