The elements of modelling in general and of performance evaluation of discrete event systems (DES) in particular have undergone a tremendous transformation during these last four decades. The aim of this paper is to look back over all this evolution, trying to retain some particular experiences from the past. I will try to classify these elements according to what I have perceived as their positive or negative potentialities. All the views expressed are my own and entirely subjective. Nothing will be proven since there will be no theorems. We first enumerate a list of events or situations which have occurred during these four decades and which I regard as positive. An opposite set of negative arguments will follow. Then, I will enumerate a list of risks that, from my personal perception, represent the dangers for the domain of modelling and of performance evaluation of systems in the field of computers and telecommunications. Finally, some suggestions to preserve the quality of the expertise of the community will be proposed.