You and I have very different definitions of success. Brannstrom has been passable at best since Chabot went down. He's had good games, and he's had terrible games. Even in his good games he still makes major mistakes (he almost cost us the game against Boston because of a horrible giveaway at the blueline). One of the top things you need in a legit top 4 dman, is consistency, you need players that you can rely on to play at the same level every night. Brannstrom has never shown he can do that. All this talk of him "succeeding" in this role is pure recency bias.
The reality is this season he's been worse offensively than 4 other dmen (Chabot, Zub, Holden, and Del Zotto have all contributed more and at a higher rate offensively). He takes an absurd amount of penalties due to getting caught out of position. And he gets outworked in every single battle in the corners due to his size. The only reasoning to keep Brannstrom around long term is because he was supposed to provide a level of offense that other dmen can't. But when 4 other dmen on the team are outproducing you (and two of them are known more for their defensive role), then you're not doing that. So what exactly is the benefit to trying to compete with a slow, 5'9 defenseman with poor gaps, and questionable decision making with the puck? I'm sorry, I wanted the Brannstrom experiment to work, but it's evident that he isn't, nor will he ever be an impact player in this league.