It may be a reasonable theory. But there are lots of ancillary industries that help support the niche - boat manufacturers, plastics/cloth companies, clothing

It may be a reasonable theory. But there are lots of ancillary industries that help support the niche - boat manufacturers, plastics/cloth companies, clothing and misc gear companies that are native to windsurfing (such as brands like Revive, Vans, Sector 9, etc are to skateboarding), and dozens of other related brands. You may also be hurting your income if you are only doing direct buys/run-of-site buys, since you don't really know what the market value of placements on your site are, you only know what the few brands you consider "endemic" value your site for. I could likely come up with a few other reasons given the time, but I think this makes the point decently. EDIT: Also, remember the incentives of the many mainstream high-traffic sites who are pushing this narrative about advertising.