Trying hard to see the rationale behind this split. Gruber's emphasis has always been on "quality," "being the best," and "being right" in everything he does. Thus the only light through which I can see this decision is his belief that the show is better without Dan. Their relationship throughout The Talk Show has had its tumultuous moments: in early episodes, Benjamin's using an Android phone drew Gruber's ire, and then later there was animosity when Benjamin would show up for the Bond episodes unprepared. Gruber would accuse Dan of "yeah yeahing" his co-hosts and contributing little in the way of earnest dissent.
On the other hand, I always had the sense that Benjamin regarded Gruber as a kneejerk Apple apologist, and this too could not have helped their relationship.
But their differences led to the show's magic. Their contrasting styles would turn the show into a focused, intimate meditation on Gruber's thoughts on industry (plus side interests of baseball, Kubrick, etc.), often serving as a preview of what written work would be to come on Daring Fireball. While it can come across as aloofness, Dan has the passive ability to subtly guide the conversation and give it some sense of structure, and he's great at knowing when to let the other person speak. There were also incredibly funny moments of East Coast camaraderie that would keep the side topics interesting and lively.
It seems that Gruber has taken all of this for granted in deciding his show is better without Benjamin. Quite sad.