The problem I have with his catchphrase is that it's used as a cop out for whenever he messes up.
Game Theory takes itself seriously. Yeah, it has jokes. But a lot of episodes, specifically the best ones, go into some really complicated areas and are trying to reveal an oversight on the part of a developer, or simply get people thinking. The "Mario is Mental!?" series is actually one of his best put together theories, because while Mario being a sociopath sounds completely absurd at first glance, especially in comparison to Rosalina being the daughter of Mario and Peach (people who don't know much about Rosalina to begin with might buy it since it's not that ridiculous before you analyse the details), it was actually a really well constructed theory that revealed a lot of things many people didn't know about, such as the back stories in the various Classic Donkey Kong games, and how they actually have a concrete connection to each other. He brought up actual obscure Mario trivia, and basically exposed a gigantic oversight in Nintendo's inconsistency with how they portray Mario. That series made Mario being a sociopath a completely valid and acceptable headcanon.
The Rosalina episode is just obnoxiously going against facts and making completely superficial observations on Rosalina's appearance, and likening her appearance to Peach more than he should be (they seriously don't look that similar, and I took some pictures of Peach and Daisy in MK8 yesterday that basically prove it through their biker outfits), and crying "GENETICS" to make the theory sound more legit to those who are blissfully unaware.
Game Theory used to be one of my favourite YouTube shows. It had the odd fumble (the episode where they claim Sonic is actually a lot slower than Mario was nearly as bad as this Rosalina episode), but in general, they had very good propositions to make.
They just seem to be making conspiracy theories for the sake of it at this point, with no solid points. Their series about the Wii U, and how it ends in "ALL GAMERZ ARE PART OF THE PROBLEM" is especially terrible, since it actually targets his own freakin' audience.
At least we still have Gaijin Goombah, who made a more legitimate theory about Rosalina and her Cat Suit. He still seems fine to me.