Most airliners already cruise at a speed that is close to the maximum cruising speed, and although there is still an element of margin available, it is probably much less than you realise.
On top of this there are other factors to consider such as ATC constraints, as well as those things you have already touched on, such as fuel consumption.
So while there is a little bit of scope in this regard, any advantages are often outweighed by the ATC requirements to maintain a filed constant airspeed, turbulence, airport arrival times, gate availability etc.