1. Infernape is hardly scary in a generation with things like Shanderaa, Burunkeru, Drizzle Politoed, Boiling Water and Urugamosu roaming about, among other things.
2. Because Sazando is utilizing both of its attacking stats with Cheer Up, less set up is needed; it can just attack from the stat where the opponent has a weaker defense.
3. Just as you don't absorb Earthquakes with Heatran or let Salamence soak up Ice Beams, you switch out Sazando in the face of a fighter.
You do have some valid points, but I'm afraid I disagree with some. Cheer Up is not Swords Dance--even though the boost is powerful, it is simply not strong enough to dent counters hard enough, I.E. Swampert if it lacks HP Grass. And I doubt Vaporeon, Mamanbou, Blissey, that grass/steel thing, and other walls will be OHKOed by Sanzando. Unless there is something I am missing (even with life orb, which cuts down on its health even more).
Infernape was a general example--I saw many new pokemon that replaced it in Fifth gen. This does, however, does not change the fact that there are several fighting pokemon, and many powerful fighting moves. A resistance to fighting is huge, and is a reason why Ghost, with an immunity, is such a great type. And therefore, Infernape is replaced by many powerful threats such as Scizor, Kojondo, Onokuresu, Lucario, some things you mentioned, and more. Thus, it makes Sanzando's durability even harder to pass off. And I don't think burn affects fire types, despite Boiling Water. And I don't even want to talk about Shaandera--obviously, everyone will be adjusting their teams so they won't be swept by it.
Whether or not a fighter is out, there are still pokemon (like the ones you mentioned) that can dent Sanzanda hard enough. Recover is an essential tool that Sanzanda can't afford to miss. If Sanzanda doesn't have the defenses to be able to set up against an opponent, then it is not very durable, and is unlikely to be a potent threat.
Note: U Turn weakness is also rather painful.