If one did not exist by design, it would arise by an evolutionary process.  It is an ecological niche in ideological space, and a rich one.  It will necessarily be filled.

Also, you overstate the case to create a strawman.  No actors need more than a short planning horizon in order for a community to persist. 

Clearly there are open conspiracies of vast scope which have been enormously successful for millennia.,  Why should there not be some which are clandestine, perforce, since their culture, their deeds, are so offensive to the mainstream that they would be destroyed but for stealth?

Clearly there are also many spurious, fantastical, or perhaps supernatural, conspiracies for which no material evidence will ever be provided, and thus which do not positivistically exist.  But the historical evidence of many conspiratorial communities is abundant, and among them some are persistent.  The degree of coordination among them is probably not great.

Supporters of a depopulationist agenda will act according to that agenda, whether by plan or by coincidence.  No conspiratorial hypthesis is required in order to oppose them in that agenda.