Are you saying maximizing load factors are worse for fuel economy or just slowing down would be a preferred cost saving measure in your opinion? Average load factor in the US is already <90%.

Either way, what about the unintended consequences for travellers if a slow down was mandated permanently?

You would need at least 15% more planes, pilots, ground personnel, etc... to handle the reduced bandwidth. For non-direct flights, fewer connections could be made likely increasing the airport population its ancillary services.

Airlines already do this from time to time usually when fuel costs outpaces pricing power:

https://www.reuters.com/article/airlines-fuelcells-kemp/refi...