Because that's how industries work. When an industry is disrupted, a new standard is introduced which ousts the previous industry standard. As the new standard becomes adopted, it develops a core following. This continues until the market hits a saturation point, at which point either the market is revitalized by a new generation of products (the most common response), or a new disruption occurs to shift the paradigm again. In the case of a generation change instead of a disruption, the core user base will grow as more of the previously new users become core users, but you will often also see a decrease in the rate of influx of new users as time goes by. There are many other considerations too, but these are the key points.
Sky Render - Sanity is for the weak.
