"RIAA doesn't need to have 'thousands' of ISP accounts closed, in fact a few well publicised account closures will have the required effect."
There have already been high profile cases of the RIAA suing people. It made them look bad and didn't do anything to discourage filesharing (incidentally, I presume you meant unlawful filesharing, as filesharing per se is not unlawful). Anyway, it may sound good in theory, but in the real world it doesn't discourage anybody.
"it shows the IP's of all the seeds and peers for a given torrent download"
This is precisely why so many torrents are packed with random bogus IP addresses, so that nobody can tell which ones are from the real seeders/leechers. This is why little old ladies get threatened for downloading gay porn.
"How would ISP's regulate a dynamic IP? Easy, after all it is they who issue the IP's and log who has which and when"
Yes and these logs have repeatedly been shown to contain errors.
"I would have thought that many ISP's would be only too happy to rid themselves of a 'serial' downloader, I doubt they make any money from those accounts anyway."
Of course they make money from these accounts. Those subscribers pay, er, a subscription. Why do you doubt it? Do you think they get free connections? In fact, those who use massive amounts of bandwidth pay more than most for their account.
The real issue here is that it should not be lawful for an unregulated non-statutory body to impose its will on an ISP. It certainly should not be lawful for an ISP to disclose my personal information to them, simply because they say so.
If the RIAA or BPI think they can prove that I'm unlawfully infringing copyright, they should pursue me in a civil case for copyright infringement. If not, they should just piss off instead of trying to find me guilty by suspicion.