Well, Mearls ran through most of the material he originally was intending on talking about on Happy Fun Hour... how to create new subclasses and the like. He end up doing new subclasses for almost(?) all the the classes in the game, as well as entirely new classes like the Warlord. I also want to say (although I could be misremembering) that one one of the last ones he did he said something along the lines that the show would need to change a bit if it was to continue since he did go through most of the stuff he originally intended to talk about.
Now that being said... if we want to go all conspiracy theorist here, Mike hasn't done a new HFH since December, he stopped DMing the Heroes of the Vale show after like 2 episodes(?), and he hasn't posted on Twitter at all the past month (with the last one being his post about Zak). I also don't recall seeing him in any of the other D&D streams or YouTube videos recently either.
So on the one hand he could just be really busy like he's said... while on the other hand, there might be some behind-the-scenes stuff going on that we just haven't heard about and he's been "off" for the last month. Whatever might be going on, it hasn't been so monumental that Nathan Stewart has had to make a statement about what has happened (like if WotC had fired Mike or something)... but there might be some internal wiping down of the walls going on right now by the D&D Dept/WotC and/or Hasbro.
Speculating is kind of pointless because we haven't a peep from anyone about anything so who knows what may or may not be going on. But I do think that Mike being completely radio-silent since the 12th of last month is curious.