Randy represents Matt and Trey trying to reject the idea that the new generation is coming, and unfortunately, they're not part of it. They try to explain the degrading quality of their episodes with Randy trying to adapt to the new generation and failing, because all he ends up doing is shitting his pants on stage, and he doesn't "get" that the tween wave music doesn't actually sound like shit to the new generation. Basically with the Randy element of the episode, they're saying that Matt and Trey can no longer adapt to the new generation.
Sharon represents the viewers of the show who become outraged at the show becoming less entertaining and well-plotted or at least funny, and become even more angry when the show tries to adapt to the current generation (when she storms in on Randy's act). Randy (Matt and Trey) and Sharon (the viewers) realize that they can no longer see each other because over time, they've both changed (as per their conversation at the end).
Stan's story takes place for two reasons. To show (I think) that the show's coming to an end, as he is a vital character, and to explain that even though the viewers don't like the show anymore, it's no one's fault, it's inevitable to grow apart from the things you've once loved.
I've watched South Park since I was a kid, so I personally appreciate that they'd do this episode, and acknowledge the direction the show is taking, and that they'll end with some dignity to realize that fact and not continue for another 10 years and ruin themselves, just like the Simpsons did. It did make me sad to realize that personally I'm becoming Stan-like in that I don't even watch tv anymore because its not the same to me. The same goes for movies and just the world in general.