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As stated pretty well above. Let me re-state for fun:
On a 100% private tracker, a ratio system of 1:1 minimum (especially on each individual torrent) is basically a pyramid scheme. At some point, a bunch of newbies will be stuck at the bottom of the Ponzi and won't be able to make ratio even if they seed for years.
Therefore, the system must be designed with 'escape routes.' Several were mentioned above, in addition to how baka does it. I'm really impressed with how well the hybrid system works here.
What happens here, and is effectively a requirement for indefinite availability, is that a pool of "superusers" put some of these torrents on their seedboxes and never delete. I've run across torrents that are up with users who have 18,000+ hours of active time. Do that math.
However the system is designed, encouragement of community members to dedicate themselves in this way is the fundamental thing that drives success. I've connected to unusual, obscure torrents here and always, without FAIL, have been able to download, even if it's only one or two seeders at 10-20 KBs. This is the first torrent community I've ever been in where I've never had the need to make a reseed request.
Especially when I run across anemic torrents like that, I reseed to 10:1 minimum, often 13:1 or 15:1, before I retire it, even if it takes six or eight months or more. I check my Inactive list regularly. Whenever I see torrents hanging out with 3 or 5 or 7 seeders regularly, I queue them back up out of archive and regularly re-start them, usually when I notice the seeds have dropped to 2 or 3.
Here's the crazy thing: they are rarely active and don't seriously impact my bandwidth. No need to choke them because they get leechers so rarely. I've collected about 100-120 torrents of this type (so far) that I never delete off my seed drive, and add one or two every week. And, because I'm one of the only three or five seeds on some of these, over time I've built up stupid rations of like 30:1 on "dead" and "anemic" torrents ... cheers to the success of a semi-public tracker system.
It's really an education problem: teaching n00bs that it really doesn't cost them anything to keep older, anemic torrents alive.