There was certainly a wave of premium cabin purchases once people began traveling again and had both a build up of cash and the desire to travel, but DL made its intentions with respect to FCM and selling more F seats clear even before the pandemic. Since the pandemic, DL has just continued to get more aggressive with with its FCM efforts to monetize premium cabin seats. Other airlines are following suit - my comp UG percentage as an EXP on AA isn't overly impressive for the times I haven't bought up.