For many years, I had run a site on Drupal 6. For some reasons, continuing was no option, as was migration (upgrading). By the way I have to thank Drupal 6 for, all in all, forgiving me more than one basic mistake (my first Drupal project ever) and serving pretty stable along some five years. Which is good. Despite this, I am happy that this performance hog now sleeps (almost) for good.
With increasing user counts, desire for more features grew constantly. Soon after brauherr.de’s first launch as a portal, it became clear that complex projects as this one need either good patronage or far more volunteers. For a good-will project with, if at all, some educational benefit this leaves few alternatives to an open source system, ideally one with a vast contributing community.