I haven't watched in a couple years, but to me the decline started when he stopped the LIVE episodes and started catering to his super rich friends. The only way he could ever get Elon, Kanye, Trump, and others on there, was to not be live so they could heavily edit and agree to not talk about certain things.
He turned into Joe "Money Moicano" Rogan, and sold out to the elites that are raping this world and humanity, so it makes sense how fewer people are watching him. Most people under 25 years old either hate Rogan or have no idea who he is, and a lot of his older crowd moved on when he changed all of his views and opinions for money and plays. It's almost like he realized the majority of the billionaires are arrogant nepo baby Republicans, so he changed all of his views to become friends with them and profit off of it.
He's also just not an interesting person anymore, the last time I regularly watched his show, EVERY podcast would turn into a criticism of COVID, or California, or "the left", like he isn't a shill that sold out for money. He's a loser manlet.