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:
When load factors get too high, recovery from irregular ops (weather, crew, or mechanical disruptions) becomes a nightmare for passengers. People end up stranded for days without available seats.
slowing down 5% adds 10 minutes flying time to a 3 hour flight. Drag varies with square of speed except when you get close to Mach 1 where it gets much worse.
Its also possible to fly 20% (36kft vs 30k) higher where drag for the same speed will be lower. Drag varies linearly with density.
An aircraft purposely designed for low fuel consumption could save 50% or more jet fuel without being significantly slower.
An 85% load factor still means you probably have a passenger sitting either side of you and a baby wailing somewhere in the distance, but now you're paying more money for your seat and arriving later.
If you want to save fuel fly 5% slower and save 10% fuel.