Why do airliners on autopilot climb during cruise?
Climbing further while cruising is saving fuel, not wasting it.

First, every airplane has an optimum angle of attack for most economical cruise. If it loses mass (due to fuel consumed), it needs to adjust either its altitude or its speed to keep that angle of attack. The goal is to keep dynamic pressure, the product of air density and the square of airspeed, constant. Since you want to reach your destination in the shortest time, climbing into less dense air is the better option.

Secondly, gas turbines need less fuel per unit of thrust when flying in colder air. As long as the plane stays in the troposphere, it will fly more economically when it flies in colder air.