An anonymous posting on a blog is *not the same thing* as a musical piece that required a lot of talent and upfront costs to produce.
Now why would that be? You're probably inferring that the anonymous posting has no value or hasn't been worked on. But an anonymous posting can be as valuable as music. If you take Slashdot's own sort of news, that's pretty interesting. It amounts to a curated collection of news with commentary, and journalism can be argued to be more socially valuable than music in our current information society. Also, if we're talking about any kind of anonymous posting, yes, it can have value. Including literary value. If Pynchon decided to post Gravity's Rainbow online, would its value as art be diminished? I think I get what you're trying to say, but the way you chose to differentiate data is too arbitrary.
Plus, you seem to be overlooking that fact that "large groups of people agreeing with you" can be made of "individual acts of defiance", and that major economic shifts in point of view take time. We're not going to snap out of the economics of scarcity overnight, but the very existence of Creative Commons is a sign that the opinion you seem to be arguing against is quite widespread. I don't really know if it's on the rise, though I'd wager it is.