Let's go thru this one more time...
Akadama does break down over time. That's one of it's attributes! (Which means it a good thing!)
When fresh, the akadama in the soil mix provides lots of air space between particles. And it holds some, but not a lot of water. Dryer soil (within reason) encourages roots to grow, to seek out water and nutrients. As time passes, the akadama breaks down, the roots have grown into it, and have multiplied. Leaves (needles, whatever) are growing, and demand more water. Broken down akadama retains more water than fresh akadama, so the water is there for the growing plant.
Also, while akadama does retain water, it's not organic. Which means it won't rot. A material like pine bark will rot. As organic material rots, it consumes nitrogen, robbing the tree this nutrient. Akadama, doesn't since it's not organic.
In short, akadama changes it's structure (from balls to being more like sand) as the root system of the tree changes from pruned to ramified. When completely broken down, it slows the root growth, because it stays wetter longer. Roots don't grow as much when they're constantly wet.
So, "finished trees" will do fine in akadama since we are really just trying to maintain the style/shape they're in. Trees can often go several years between repottings when they are in akadama.
Younger, developing trees will benefit from more frequent repottings into fresh akadama, to stimulate rapid root growth, and vegetative growth.
Truly, akadama is wonderful stuff for bonsai. It's almost as if it were designed to be bonsai soil.