@DjerbaDevil
There are a couple of points where you misunderstand:
1. The VAACS model is not predicting concentration. It explicitly stated in the earlier graphs (although appears to have stopped doing so) that density of the "cloud" is unknown. This is not to excuse the process, but merely to state a fact. Maybe the ICAO should alter their process to not consider such models where concentration is unknown, but the VAACS model is officially approved by the ICAO.
2. The VAACS model does not try to suggest at which concentration of ash engines might be damaged. That is for the engine manufacturers to decide. Since they do not know, they recommend concentrations of zero.