Adult 2.0, of course.
People aren't going to get tired of sex on the internet. The only thing that can possibly every be "over" is particular products and revenue models.
Now, let us assume the impossible -- there is nobody left on the internet with adult interest. The anti-sex warriors have triumphed and we live in a world with a forced salt-peter diet where babies are manufactured by Ford and General Motors because nobody can be arsed to stick a dick in a pussy.
OK. What do I do now? I create what the internet marketing theorists call "traffic of intent". And I sell it, various ways, to advertisers.
That's a lot of fun. If I weren't doing it in "adult", I'd be doing it anyway. It's a great business, no matter what the topic.