If you are too high on the G/S (or want to descend etc), you reduce thrust.
If the plane is in trim, it will keep the speed
Vice versa if you are too low / want to climb.
Hence pitch controls speed and thrust altitude
These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.
However, if you want to change the speed you use thrust to change speed and pitch / trim to maintain altitude. But this is not a constant system or stabile environment (and not the way an e.g. an ILS is supposed to be flow).