No, what they should have done is exactly what other airlines do when they are overbooked: keep raising the voucher amount until someone volunteers to get off.

No, what they should have done is exactly what other airlines do when they are overbooked: keep raising the voucher amount until someone volunteers to get off. Or do what European airlines are mandated to do by law: Offer cash! I, for one, am absolutely not interested in any vouchers that probably come with a shit ton of fine print, which makes them either unatractive, or impossible to redeem. Or probably both.