Yes, it is true that each time you wash, the surface of the paint will appear less and less shiny. This is because both real Carnauba wax and synthetic waxes slowly deteriorate from the sun’s rays, weather conditions and the fact that you are ‘abrading’ the surface each time you rub anything over it. This includes a wet sponge, rag, wash mitt, brushes, etc.
In addition, you are ‘abrading’ the wax as you dry the water off the car. The car will look it’s shiniest right after you wax it.
If you left it in the garage and never took it outside the wax would look good until dust covered it. Simply wiping off the dust with a cloth will also abrade the wax and will cause a slight reduction in the shine. If you kept the car in a dust free environment but just ran a soft cloth over it each week, that too would slowly diminish the original shine created with the last application of wax.
Unfortunately, despite claims of some wax manufacturers, there is no magic or secret method of keeping a car looking freshly waxed other than waxing it often.