Any Legendary member (which OP referred to specifically) can make way more than $20/week, and I don't see them as being the problem with shitposts--maybe the drama threads, but members who've been around a few years tend to make interesting posts, if not always substantial ones.  It's usually the people who come here for the sole purpose of earning money through bounties that are the problem.

But anyway, I haven't seen any major decline in bitcointalk in the past few years.  It seemed to be waaay worse before the merit system was implemented, and that was over two years ago.  I'm also pretty sure the real decline started when signature campaigns took off, but that was around 2012 or so if I'm not mistaken.  It was certainly already bad when I registered in 2015.

How long was your break?