Some of the blame can be laid at Dan's feet.
Early on, he was always super quick to apologise for any joke that was even slightly risque. He would usually play off his apologies as jokes, in a sort of inside "we would never actually say that for real" kind of way. This is speculation here, but I think Dan doing that all the time made Arin either uncomfortable or just more self conscious of the 'message' he was putting out there when he made jokes. Most everyone likes to say Dan is nonconfrontational and Arin is the envelope pusher, but when it comes to his friends (Dan, in this case) I think Arin prefers to take the road of least resistance as well. Eventually, the only jokes either one felt fully comfortable with making boiled down to sex jokes and toilet humor. As time has gone on, Dan has checked out more and more to the point where it feels like he just doesn't care. Arin has amped up the tard rage and idiocy to compensate, but all it's done is make him look, well, fucking exceptional.
More speculation here, but when GG started with Jon and Arin, the audience was mostly young to college age males. As of 2019, I'd say the audience is mostly young male children/teens and college age/older females. Kids tune in for the poopy dick jokes while the ladies listen for the weird innuendo and the creepy fantasy they create for themselves with Dany and maaaaybe Arin.
It's not the show we started watching for, and it never will be again. All we can do is just out the popcorn and take bets on how big the final explosion will be.