Some new posters do commit non-cardinal sins, like resurrecting ancient threads or drive-by posting; and some are cadinal sins like trolling or advertising/spamming.
First, the practical difficulties:
(1) Without some sort of monitoring, a 24-hour holding room doesnt seem to me to solve any issues.
(2) Mods don’t have time to review all new posts. Even if we did, yeah, we’d catch zombie threads (if we read carefully – we can fall into the same trap), and we’d catch spam/advertising a little earlier, but we wouldn’t stop the trolls because we usually can’t detect them until after several posts. (Just because someone holds an unpopular opinion doesn’t mean they’re trolling.) So, basically, it’s a lot of work for not much result.
Second, the conceptual difficulty: we need to attract new posters. Frankly, it’s difficult enough for new posters, there’s a certain cliquishness here. Note that many posters respond to an old thread revived by just saying “zombie.” The new poster doesnt understand what the heck that means; unless there’s kindness and explanation, that person is going to leave. (Don’t tell me that they can read the specialized vocabulary and the rules; most new posters don’t and don’t want to do that much work, they just want to see what happens when they post.)
I, frankly, think it’s terrific that a new poster has gone to the trouble of finding the relevant thread to post in, even if it’s a few years old. New posters typically come here because they’ve found something (Cecil’s column, or a discussion topic) that they want to respond to. Hurray for them, and we should be encouraging, not discouraging.
If we discourage new posters, we won’t last long.
I do agree, it would be interesting to compare first posts (or “flurry of activity”) with later activity and sticking-around-ness, but that also would be a lot of work.
So, the bottom line: we don’t think zombie threads are that much of an issue. In several forums, they’re not an issue at all. Instituting some sort of monitoring system would mean LOTS of work, for very little results. I think the expression is, “Using an elephant gun to shoot a fly.”