I'm not sure about this point of view that "no sales = dead system". I can understand the reasoning, but it's definition of dead is very narrow. What do you think? Is this a large part of what decides if a system is dead? Or only one point of view?
Given that people who started playing popular RPGs (e.g. D&D, Rifts, WoD) after the early 1990s are used to lots of support material published on a regular basis and have come to depend on that stream of published material for their adventures and campaigns, lack of support material probably does mean "dead game" to them. People who started playing before that or who play less popular systems are probably less likely to think that way as they never had a steady stream of supplements and adventures to depend on and so are used to creating their own.