Greetings,
We can debate for centuries, however what happens in the real world?
Tomorrow within a 100% users prefered trajectories, block altitude airspace then lower speed to glide at idle thrust (still debatable fuel Flow) then saving 20 nm to 30 nm from cruise segment, tranformed into descent segment, yes you will save some fuel providing that there is no error on your TOD position.
Today different ball game, you cannot do what you want in the airspace, and ATC manages their airspace as per their rules and creteria.
Cost index or no cost index it is a fact.
