Leaving without an agreement can only mean that both sides have not yet decided what they want to get.
That does not make sense, could you elaborate please.
The problem can be created for those who will make air flights (it is necessary to mark the air borders). I think they need to reach common compromises as soon as possible.
Air travel will be just one of the many intricate and complex issues need resolving. This includes agriculture, fishing, trade (including VAT, customs, excise, import/export), banking, intelligence sharing and so much more.
I disagree with you, they do not have to reach compromises as soon as possible, the status quo suits all parties because they have until 31st December 2020 to reach a deal. Most analysts say an extension will be made for another 3 or 6 months after that date to finalise any outstanding issues.