Kyurem-B has a massive Attack stat, an impressively usable SpA stat and fearsome bulk. His movepool may not be as fancy as Dragonite's but he has all the tools that he needs. Kyurem-B is very well known for being able to go Banded, SubKyurem, or Mixed. This alone places an advantage over the aforementioned Haxorus, as Haxorus is monotonous and straight-foward in function. Kyurem-B's offensive prowess in conjunction with his impressive bulk seriously allows him to threaten any team, offensive or stall. Just because he lacks Earthquake doesn't make him a lower-ranking Pokemon. He has Earth Power at his disposal which allows him to dent those Pokemon that you mentioned. His Dragon Tail is also pretty strong and while it won't KO a Steel Type, it will seriously wear it down with Hazards in tandem with a possibly Boosted Dragon Tail from Hone Claws. Kyurem-B also has possession of Tailwind. It's surprisingly dangerous and can make that Terrakion think twice about switching in. All in all, Kyurem-B definitely is A-Rank material. I'm kinda wondering if something with that much power and coverage will stay OU for long. If so, I want to see it rise to high-A or even S-Rank, cause when we talk about power, everything else, like his boring and small movepool, simply just hold little to no relevance as he will be smashing any Pokemon willing to talk in.
The fact that you said Infernape was useful in certain situtations just goes to show why he isn't S,A or B-Rank. His niche is very small and to make things worse, it's very situational. Being able to speed-tie with Terrakion isn't really a niche as well. And if it is it isn't very impressive as the other musketeers can do the same as well, not to mention the Lati twins. The SkarmBliss core still holds tried and true but it's been getting rarer and rarer to encounter. And once again, it's a small niche. Kyurem-B, Terrakion, Lucario and others can fulfil that same niche that Infernape holds. In fact, they can do that niche and more which allows them to outshine Infernape. Honestly, C-Rank is pretty solid for the Ape and unless something happens, Infernape isn't looking to be climbing up that ladder anytime soon.