It's a bit like any legalization question -- the black market exists anyway, so a regulatory framework could bring at least some of it into the sunlight.
But that'll only stop a small part, anyone could share the infohash and if you're using the dht/magnet without .torrent files or clicks on a website, no one can count those downloads unless they too scrape the dht for peers who are reporting they've completed the download.
Which can be falsified. Head over to your favorite tracker and sort by completed downloads to see what I mean.