It was a funny episode tainted by the very bad writing and characterization. Fat! Barney and Old! Robin are just something I can't wrap my head around. It totally came out from nowhere. Barney coin-cheated to *not* have sex? Please. Even in a parallel universe, not happening dude.
Unlike Ted-Robin or Marshall-Lily, Barney and Robin weren't given half a season to grow after all the build-up. Of course, the writers might set this break up for a BroTP reunion later on, but I can't see why they ever want to get back together since there is no real problem to begin with. Apparently they make each other so miserable Barney resorted to binge eating and Robin stopped washing her hair. Them being too awesome isn't something they could do anything about anytime soon imo.
I've always believed that Barney and Robin can just be as awesome and even more so together. As awesome as he is being single, I don't want to see his character not developing at all. What people have been complaining about this season, I would say, is largely due to poor writing than the fact that being in a relationship dulls his character.
Maybe the writers are pressured to please fans who dislike the new Barney, or they just want to sell the playbook that is due to come out this Christmas, the kind of book that won't be penned by in relationship Barney. At least, they could have written a more believable break-up.