Yeah.. but not really. The non-public domain documents would still be copyrighted so nobody without a license to them would be creating a JSTOR competitor anytime soon.
Furthermore, torrent of PDFs would never replace a subscription to JSTOR for any university. Universities that currently subscribe to JSTOR would still be subscribing to JSTOR, the only difference is that unaffiliated individuals would have other options. How much of JSTOR's operating budget is coming from selling individual articles to people not in universities?