They don't care even when they are wrong they continue their wrong doing and do not give a fuck about what really is going on in the forum, as long as they are making money off of their DT status they don't care.
I would love it if you would name names here, though I doubt you're going to do it. The DT members I see who are actively tagging scammers actually do care about the forum, and as to the part I've bolded above I'm really curious who you think is making money from their status as a DT member.
I sure as hell haven't earned a penny since I've been on DT, and it's been mostly a headache from the complaints and the "work" I do is on my own time and is precisely because I
do care about improving bitcointalk. When I tag people, I try to get it right but sometimes I don't and when evidence presents itself that's contrary to the neg I've left, I'm not above swallowing my pride and removing the neg. I'm speaking for myself here, of course, but none of the other DT members active in tagging cheaters are earning any money as a result of that as far as I know.
I would be against scrapping default trust, as I think it is probably most useful for protecting newbies and junior members who have not yet decided who they would and would not trust themselves, and these junior users are the the ones who are most likely to be scammed.
I think DT serves a purpose, but you can't rely on it too much, and I agree that it shouldn't be scrapped altogether. Some changes might be in order based on what you wrote about some DT members not being active for a long time. Those are DT1 members, though, and that's Theymos's call.
I think people ought to create their own trust lists after they've been here a while and figure out who
appears trustworthy. There's never any certainty about that, as I've seen multiple DT members lose their status for a variety of reasons, and anyone can either go rogue, get hacked, or sell their account to a scammer--you just never know.