i feel like homestuck began to decay was when the whole "skip to the trolls" sentiment started, it may just be me, but skipping a HUGE chunk of the story just because you don't think it's as interesting as "huehue silli alien relationship drama" is insane???
plus like... does act 5 make ANY SENSE without the previous 4? personally i've always been able to make it through those acts real fast, there's not a ton of text, it's easy to read and it mostly tells the story through subtle hints on the pictures!
which is why i'm sort of pointing out this was the "start of the decay", the first troll sections are mainly what birthed this idea that homestuck was overly long dialogue and expository text, instead of fun and wholesome character interactions or awesome illustrations with lots of lore details and silly pictures of the characters being funny, this idea ended up getting carried over to act 6 and it became sooooo much worst in post canon to the point the epilogues didn't even have any damn pictures! plus it based itself just on the melodrama and pretentious narrative that was never a part of homestuck and people just made up to feel smart.
mind you, I'm not saying act 5-1 is horribly bad or at fault for it, at the time it just felt like a bump in the road because act 5-2 manages to bring almost everything home spectacularly, but mainly that the misinterpretation of homestuck as a whole and the idea that acts 1-4 are unecessary and you should just "skip to the trolls" because THAT'S where the interesting story is is what ended up causing the themes and narrative to switch towards focusing on relationship issues and melodrama in act 6, then to overly complex metanarrative and annoying philosophical discussions in post canon that came out of nowhere.