I don’t think this is true — every airline that I know of supports “seat assignment at gate” to handle exactly this kind of overbooking.
(How it happens can vary: your PNR might not have a seat assigned, or the seat might have been double booked. Either way, at check-in they’ll typically notify you and let you through regardless, since you have a perfectly valid ticket.)
(How it happens can vary: your PNR might not have a seat assigned, or the seat might have been double booked. Either way, at check-in they’ll typically notify you and let you through regardless, since you have a perfectly valid ticket.)