Obviously branding matters, I just don't believe branding alone can account for the success we currently see.
It can account for the vast majority of it. You completely fail to appreciate the margin of difference in how much D&D is known compared to all other RPGs.
You bring up DC. That's a poor comparison on multiple levels (
@Morrus illustrates another level). Imagine if there were a situation where most of the public, including young people and nerds, like fairly serious nerds, only knew about DC comics heroes. Next would be Marvel which a micro-generation from the '90s had a few people who knew about it, but wasn't very popular.
That's where D&D is. Nobody outside of ultra-nerds and a micro-generation who has some familiarity with World of Darkness knows about other RPG. Even Pathfinder was basically only known to people who were existing D&D fans and thus ultra-nerds. Even when journalists mention other RPGs, they describe them with reference to D&D. I saw an example of this with Joss Whedon trying to explain how Firefly was inspired in part by what seems to be a Traveller campaign, but the journalist didn't even record what the RPG's name was, and just explained that it was a "sci-fi RPG" and referenced D&D to explain what an RPG was.
D&D has
no competitors brand-wise. No competitors for mind-space with the general RPG-playing public. Given how popular D&D is right now, another company, willing to spend tens of millions, could probably nab double-digit percentage of D&D's market share with a sufficiently well-produced and marketed RPG. But they'd have to spend
tens of millions marketing a pen and paper RPG, and there'd be absolutely no guarantee that they'd succeed. It doesn't help that where projects might stand a small chance of making some ground, where they do breach the "mid-nerd" barrier from the aphotic ultra-nerd depths, at least temporarily, are often badly handled. For example, the new Marvel RPG, which would potentially stand to make some ground, is absolute trash-fire mechanics-wise, extremely crunchy, badly-designed, not enjoyable to play (by most reports), and generally godawful. It's a great design if you want to ensure anyone coming from outside RPGs or from D&D goes "Uhhhh no thanks".
With a lot of IPs, that would mean people would just shoot that product dead and pass the licence on to someone else. But instead, because it's an RPG and there was never any serious expectation of making money, Marvel has done nothing.
But anyway, point is, D&D is virtually a monopoly. It doesn't operate in a monopolistic manner nor attempt to harm competitors, because it doesn't have any, it's just a blue whale compared to a bunch of remoras. Even Pathfinder now is, at best, a dolphin (even at its strongest, all it did was take half of D&D's audience away, it didn't grow the audience of RPGs at all).