I've actually given this topic a lot of thought over the years, and here's my opinions. Firstly, I personally don't even look at 95% of the threads anymore, because I know that they have been answered (either newly or in the past) and don't interest me in the least. Secondly, bringing back up an old thread from months or even years back can be a little weird, too. Users that used to be very frequent posters may have moved on, and you can't ask them for clarification of their posts anymore, or it is even possible that the information is inaccurate because something new has been learned recently. Thirdly, I think that having a lot of threads going is an indication of how active the forums are, and even the newest beginner can probably find some threads they can participate in. I think it is easier for a beginner (to either fishkeeping or forums) to participate in a thread with only a few posts in it than a full page. And lastly, not too many people read an entire thread when it gets to be more than 3 or 4 pages long. Which is a shame in most cases, but you can tell when people haven't read an entire thread when someone posts about an issue raised and put to bed 40 or so posts back.
So, I guess in conclusion, as long as there aren't 3 or 4 or 5 fresh threads about the exact same topic going on, and the posters have read the stickies (that is, the post is not one that would have been easily addressed had the OP read the stickies), then I don't have a huge problem with it. But that's just me, I'm sure the opinions on this run the entire spectrum, and I can see the benefits of trying to keep everything in as few threads as possible, but with so many users and new users it's going to be much harder.