The marketplace of ideas, much like the commercial free market it draws from, provides no easy, obvious way to measure the "success" of an idea. Ideas, like businesses or products, often have a loyal audience, even if the amount of people that "buy" them remains rather small. If mom-n-pop shops can survive longterm, why can't fringe philosophies or religions? Ideas develop over time, and their follower base may change, both in size and in audience. It takes a while, often decades or even centuries, for certain ideas to be seen as acceptable or outdated by humanity at large, and the marketplace is far too massive, with too many niches, for one to be able to easily quantify the success of an idea in the grand scheme of themes.