Snark aside, 4chan was once hugely influential to internet culture and humor, but that era was long, long, long ago. When 4chan became the breeding ground for the alt-right, that thoroughly infected the site so badly that everyone else was driven off. There is nothing of value left on that site, and its death is a net positive for the world.
Of course other imageboards still exist, but I don't think any of them will ever be as big as 4chan was. I hope that none of them become that big.
'Containment' doesn't work. That was the reason /pol/ was created, but all that did was rapidly radicalize the userbase, which then spread back to the other boards.
Giving them a dedicated space to radicalize people doesn't mean that the people who get radicalized will never leave that space. Because not only did it spread back to the other boards, it spread outside 4chan. The rise of the alt-right can be directly traced back to Steve Bannon recognizing 4chan as the ideal target to start radicalizing users there. And now look at where the alt-right is today. It's not an exaggeration to say that this is a large part of how we got Trump.
'09 was the year the right-wing postbots took over. It was a good time to leave. It's also the year after Paul Manafort received millions of dollars from a Russian oligarch to promote Russian interests in the West, and the same year the Tea Party was formed.
Paul Manafort was also on Trump's top campaign staff in 2016.
Even if it is over a decade late, we should pour one out for old 4chan. It was never a great place, but what happened to it is a huge shame. If this really is the death knell, I wanna remember it as the weird anarchic slice of the Internet it was, rather than the Nazi bar it got turned into.
I think 4chan had been largely superceded by reddit as the min meme source. And now tiktok et al is largely superseding reddit as reddit continues going to shit.