You bring up many good points. I have for years wondered what the music and movie publishers are thinking. Yes, online sharing is rampant. Naturally it is, what other way is there?
I can't be the only one to suddenly realize I'd like to hear a certain song, see (part of) a certain movie, show or documentary? And how can I do this? Well, I can go to Kazaa and download it, or... Well, that's it. The publishers give me no alternative.
I have no idea what the average CD or DVD buying rate is, but I suspect it's less than 1/month on average. It certainly is for me. But I'd be happy to pay 20 bucks a month to have access to a large library of movies and music. And note, those 20 bucks would go straight to the publisher, with no cut for retailers and distributors. (There'd be a certain cost for providing the service, no doubt, but I'm sure the publishers would end up with more of the money than when they sell me a CD/DVD.)
But what if I share what I download with my friends? Well what about it? If the service is convenient and reasonably priced, the only ones with an incentive to get things from friends would be those who could not afford it anyway - no loss there, just more incentive to get the service when they can afford it.