Unlike boarding, when deboarding one has a different situation.
- all pax know exactly where they want to go
- all pax are already in their assigned seat
- pax do not have to find info on their assigned seat
- nobody needs to find a place to put their luggage
- pax are going from somewhere cramped to somewhere with potentially more space. Farts obey Boyle's Law... pax do so as well
If all pax are allowed to get up the second the seatbelt sign is turned off... then you have lots of people retrieving their luggage and putting on their coats *at the same time* - ie we have parallelisation. If pax can stand up only one row at a time, you have to wait for the suitcase retrieval process to be done maybe 30 times in sequence... which translates to longer deboarding and longer turnaround.
Yes, an airlines wants to let pointy class pax off the aircraft first... but why would an airline, particularly a LCC whose business model requires high aircraft utilisation, want to deboard economy class pax a few rows at a time, when they can't claim any sort of Covid moral superiority if nobody is wearing masks ? Longer turnaround times are rarely desired by airline management