The big problem for rightsholders with copyright infringement on the Internet is scale. In the anonymised but highly connected world of today's Internet, one site hosting infringing content can easily supply it to thousands of people within a matter of hours, and one P2P network can involve numerous people each helping the others to share infringing copies without even knowing who those others are.

The thing you keep ignoring is that this only works because of the public but anonymous nature of sharing material using these tools. You can't cause damage on that kind of scale if you're only sharing between a few friends that you can immediately identify, and the low level of infringement that such an arrangement might support is little different from the old days of trading music tapes in the school playground.

In practice, the realisable value of a major copyright-protected work, the kind of thing that costs millions to make and sells very many tickets/copies/whatever, is mostly collected within the first few days anyway. If a DRM system on a game holds off mass infringement for even a few weeks before the cracks come out, it's probably achieved 90+% of its potential. If a movie studio can keep a decent copy of a new blockbuster off the torrents even past the opening weekend, that's a huge benefit. Similarly, if rightsholders could reduce copyright infringement to something that really was a social activity rather than a massive industrial exercise, the benefits would be enormous even if that low level of illegal copying was still going on in the background.

So, all they have to do to defeat all these hypothetical future scenarios of yours is sign up for the same public but anonymous networks, record who is providing and/or downloading infringing material via their node, and then bring legal action against them both to reclaim their costs and actual damages and to identify the next links in the chain. Given that at this point we are talking about people who are known to have been breaking the law, I don't have a problem with the legal system providing for modest and realistic assumptions about actual damages but also awarding substantial costs to make these kinds of investigations and legitimate legal actions financially viable. Given reasonable safeguards to protect genuinely accidental/occasional indiscretions, I also don't have a problem with somewhat heavyweight actions like enlisting the help of ISPs to trace content that is now known to have been infringing back to its earlier sources, or imposing penalties on services that are known to be used for say anonymising transmissions but refuse to co-operate when an illegal action has been identified.

It will be harder to do this than just cutting off a handful of obvious, high-profile targets today, of course, but just as no DRM system can completely protect a movie that you can actually watch, so no distributed P2P system can completely anonymise where traffic came from. Just make sure any costs associated with these additional burdens are borne only by those who are found in court to have broken the law, and you probably have an effective deterrent that really is based on actual damages and measurable costs rather than absurd artificial estimates.