lol alright I gotta admit that was a crazy error on my part, guess I clicked Charizard when looking for Mega-Y. Still though, as impressive as that 1v1 potential is it's certainly not an ideal situation. I'm also seeing more protean Gren's running rock slide which I'm assuming is a one shot. Furthermore since we're discussing Infernape's viability and not so much Zardy's I'd like to point out that Infernape does have a niche over Zard in that it has a greater speed tier, neutrality to stealth rock, and the ability to hit stuff like Chansey and Toxapex. Honestly it's probably a decent teammate for Zard Y. I wouldn't mind seeing infernape in like B- or B, it's above Pokémon such as cofagrigus for sure.
I feel like if you'd run Infernape, you'd have to run either LO 4 attacks, Scarf or SD, which both have it's enormous drawbacks. If you run LO 4 attacks, your attacks most likely have to be Blitz, CC, Mach and then U-Turn. With this set, you're completely walled by Toxapex, can't break through defensive Lando, defensive Mew CAN switch in on it and Roost up, and if you attack Zapdos w/ Blitz, you have a chance to get paralyzed. Also, if you're running EQ > U-Turn/CC, you're still not breaking through Pex.
252 Atk Life Orb Infernape Earthquake vs. 252 HP / 192+ Def Toxapex: 135-161 (44.4 - 52.9%) -- guaranteed 3HKO after Black Sludge recovery
SD 'Ape is cool, but still gets walled by Pex who can Haze, and you have a hard time setting up against most threats in the meta.
Scarf can be a cool revenge killer, but is predictable after the first time you attacked.
Infernape definitely does have its niche though, but it's nowhere good enough to be in the B ranks. I'd say C-/C as a start.