See, I'm a brand main, but I'll try my best to be unbiased in how I respond to this thread.
I can see where you're coming from, his passive does very high amounts of % health damage. However, his damage is very situational, and can easily be avoided. In order for Brand to even proc his passive on one target, he needs to land a full rotation of spells, that becomes extremely difficult as your opponents get better. His pillar is easy to avoid, his stun is easy to juke, his E almost requires you to be in auto attack range and his ult is situational. Brand is like Yasuo, he forces you to break your "safety zone" that we all learned as a new player. We were taught to stay near friendly minions, but against Yasuo that makes him stronger. We are also taught to "group up" as a new player, which is where Brand shines. But if you spread out and know the range of his abilities, his ult won't spread, nor will his passive. Then he's struck relying on his cooldown rotation which is about 4-5 seconds (with cdr). Now the balance comes from the fact that his kit has literally no safety to it. Most every single other mage has more reliable CC or better range to exploit, where Brand needs to be almost front line to deal damage. And because the mage update made his passive more suitable for combos, his poke is no where near as good as it was before.
As for your claim about him being banned at world's, I can see why. As a support he brings a lot of pressure, and his kit is suitable for a 2v2 lane. It's basically a counter to the way the bot lanes are being played. Also its been a targeted ban on certain Brand players that are extremely good on him.
I just feel like nerfing his damage isn't the right way to balance him, but instead either decreasing the explosion radius of his passive. Potentially they could even lower the total explosion % hp damage and allocate it back into the burn damage.
All I know from playing Brand alot since season 3 is that he's strong in teamfights against people that underestimate his damage. But when he gets focused, he has absolutely no way to defend himself.