To put it another way: If Debian were to vanish tomorrow, what would become of Ubuntu? Of MEPIS?
(Please note that I do not mean to imply that Debian's only value is as a base for the above or other distros. Far from it, I find Debian extremely valuable in itself, and have great respect and admiration for Debian maintainers. But I also believe that Debian's importance extends beyond itself to the those other popular distros based on it.)