CP will always exist. Especially if you group everybody under 18 into that category (a 17 year old is not a child, a six year old is) -- a 16 year old can't take nude pictures of herself without becoming one.
That Freenet is currently filled with it is a good thing -- it indicates how much pressure society puts on the people who are into this stuff and that no other group, such as the KKK, gun rights activists, astronomers, has that much reason to fear going public with what they right.
And any such group will quickly fill freenet up with other content, as it will be much bigger than the child abuse rings.
Most groups that would be on such networks (other then child porn producers/consumers) would be mostly political in nature. As such their use of said network would mostly be to distribute textual material (forums/blogs/manifestos/howtos/newsfeeds/ebooks/etc). It would be material that takes up less space, but is actually more time-consuming to produce. For example, writing an ebook takes longer than rolling the video camera for a couple of hours, and produces a much smaller (space-wise) amount of content.
Not really. Astronomy generates nearly endless amounts of data (although they are no longer likely targets of haressment) and the other groups would be posting plenty of video (mostly recordings of police stops, howtos, etc).
And while 16 year old sexting would be considered cp I highly doubt they would be uploading it to any such networ, if only because they don't want others to see them.
Your comment about "quickly filling freenet up with other content" seems to assume that many more people are interested in astronomy, gun rights, or the KKK than are aroused by child pornography.
Is that true? How would you know? Is there statistical information out there about what fraction of the population is aroused by child pornography? Given your statement about naked 16-year-olds sexting falling into that category, it seems implausible.