The dumbest answer to "Why has 4chan gotten worse?" has always been "It has politics I don't like."
The real answer is the same as for everywhere else online: the Internet went from a place for high-IQ white & asian nerds, to a place for low-IQ brown people. You don't need to seek out a more complex or interesting answer.
I've been on 4chan since 2005, and you're essentially correct. 4channers in those days were a chaotic and creative bunch. /b/ was an extremely entertaining board back then. Once zoomers and third worlders started piling in, the post and meme quality tanked. The current generation of 4channers are morose and tend to lack creativity.
/b/ is a grey husk of its former self. There's no creativity or fun there. It has essentially become a porn board. I know many seasoned oldfags migrated to /new/ and then /pol/ for a while, but that was relatively short-lived.
Speaking of /pol/ specifically, the last hurrah of that board was the George Zimmerman trial in 2013. The threads regarding that subject were extremely entertaining. The threads regarding the manhunt of Christopher Dorner that same year were also a lot of fun. However, /pol/ has been a malignantly useless board since at least 2015. The 2016 election sealed its fate. 99% of the posts on /pol/ at any given time are nothing but noise.
I don't use 4chan much anymore. I check /k/ and /x/ mostly, and sometimes /tv/. However, those boards are mostly just low IQ drivel now, too. Very rarely does a thread catch my attention. On /k/, I'll check /arg/ and /gq/, but there is rarely any decent discussion. /x/ is 95% garbage threads, but sometimes there are gems. There's a really fun Halloween General every year on /x/, and I frequently steer those threads by creating OPs and posting new content. /tv/ does have an ongoing horror movie general most of the time now, and it's a good time occasionally.
I've been returning to old school forums for online discussion. There tends to be better moderation and higher post quality. Even certain subreddits dedicated to my interests have better discussion and content than 4chan. That's probably the most damning indictment of 4chan I can think of.