Actually, the previous system did allow server leeching, which was actually done regularly, notoriously by
free-game-downloads.mosw.com:
http://www.abandonia.com/vbullet/showthread.php?t=13204
I'd love that slow connection users could be accomodated in as many ways as possible, but my first intuition is that making download URLs not to expire is inherently a vulnerability. I can't see a way to accomplish both things. I'm no expert either so I may be wrong.
If there is a way that users of slow connections could be allowed to do interrupted downloads, it would be by using a download manager that doesn't depend on the direct URL to persist, and at least let you enter new URLs manually for the same file. Surely that's perfectly possible.