On the subject of 'how much has 4chan declined?,' the three boards I've historically frequented are /ck/, /out/, and /tg/, and since 2 of these are boards I don't see talked about much I thought I'd share my experience with them.
/tg/ is obviously a shadow of its former self, and no longer 'gets shit done,' alas, alas. There's just too many posters, and not enough community. It's still possible to have good discussions on certain threads, and so it's still worthwhile to read, but the energy is gone. Still, this is by all accounts one of the better fates to befall an aging internet community.
/ck/ has devolved into deeply depressing nationalist wars, where every fucking thread devolves to "American food sucks" "Indian food sucks" "British food sucks" "Italian food sucks" immediately. The posts aren't even funny, they're just angry and spergy, and they make one yearn even for the days when the board was dominated by fast food posting.
/out/ is fucking shit, but /out/ has always been fucking shit. That's the real reason I made this post: to complain about how embarrassingly godawful /out/ has always been.