Those two have worked together for a long time, so it probably was a breakup of sorts. Plus everyone associated with that show are raging SJWs, so you know it has to be a drama fest. Having said that, Millar is the most entertaining stoner bro of this generation, a veritable Jay for the millennial generation. And he was dead honest about everything, including being critical of himself, so I have a measure of respect for his choice, even if I don't buy into his politics.
I do think the show is fucked without him, though, unless the Gavin character really picks up the slack. The lead character is a neurotic unlikable shitball at this point. The indian guy is the character I personally gave the least fucks about every season. Gillfoyle and Jared are both quintessential straight men, but without Ulrich they will have virtually nothing to play off of most of the time. And Jian Yang, while killing it every time, was good because he had such a great target with Ulrich. I mean, who is Jian Yang going to shit on and have it be even remotely as funny? The whole reason that dynamic killed was because Ulrich deserved to get shit on and he knew it. There is no one left to be that target that lampoons the entire culture and makes those other guys work. Unless of course Gavin becomes the new main guy and is preyed on for his social ticks by the others, then it might work. MIGHT. There is an art to being the dignified asshole clown of a group and Miller might only be able to do that shtick, but he does it better than just about anyone else ever has, with maybe John Ritter, John Balushi, Charlie Day, and Bill Murray being the only other guys who do it better.