It's totally not. People essentially enjoy creating and sharing, and love putting up quality content about things they truly give a shit about.
The internet was built on people writing massive forums posts about completely stripping down Kawasaki motorbike engines, and hosting their own websites where they ramble about conditional probability. Wikipedia was written by unpaid but enthusiastic curators, and no one got paid for uploading popular Youtube videos until long after it was already a staple of the internet.
I totally and utterly refute your statement. The world came to the internet for the content that people made for free, because it was the best. It's still the best. After all, you're here reading the comments on hacker news, which we are all freely contributing too with no expectation of compensation.
Quality online content will not vanish if online advertising fails, because quality online content is posted by people who first and foremost /want to post content/. It'll move to different kinds of websites depending on how people want to access content at the time, but it will still be here, just as it always has been. What will vanish is all the mediocre spacefilling bullshit. The stuff so dull you _must_ pay someone to write it because no one would do it for fun or kudos.