You sign a contract that requires the airline to fly on a certain date and you to pay on a specified date. The only change from today in the normal case is that the two dates are the same.
The big changes are in two unusual cases: ① The airline closes before you fly, such as Swissair, Air Berlin etc. In that case you don't pay. ② The airline cancels your reservation, as many did during the early covid phase. In that case you and the airline quarrel about nonperformance of the contract, just as you do now, but you have the money during the quarrel instead of the airline offering you a voucher.
The big changes are in two unusual cases: ① The airline closes before you fly, such as Swissair, Air Berlin etc. In that case you don't pay. ② The airline cancels your reservation, as many did during the early covid phase. In that case you and the airline quarrel about nonperformance of the contract, just as you do now, but you have the money during the quarrel instead of the airline offering you a voucher.