c. a portion of the industry is based on a delusion

Given that some industries (or movements, ideologies, etc.) could credibly be based on delusions, what do you think is the upper limit for how big such an industry could get?

The ad industry is an easily verifiable one, just go ask any of your small business running friends to see if they have generated more profit than they've spent on ads.

My business has generated more profit than it has spent on ads. ;-)