Mainstream is a term that covers so much that it's a difficult question to answer.
If you define mainstream as building a blog or scraper pages and pushing affiliate stuff through SE traffic (be it serps or adwords) then it's gonna be tough. If you look at one of the 10,000 other ways to make good money through 'mainstream' then it's not so tough.
I'd also disagree that porn pays 'bigger and better'. I get conversions of 1-50 from a decent amount of traffic for a lot of manstream stuff. It's tough if not impossible to do that with porn any more with any consistancy.
