At least infernape does have those counters who are reliable. I wanna quote what you said "Tentacruel and Vaporeon if they try to switch in multiple times" Lol, their switching in multiple times. You cant even switch in safely ONCE with salamence.
Infernape counters: Tentacruel, Vaporeon, Starmie, Suicune, Zapdos, Gyarados, Salamence, Dragonite, and a few UU poison pokemon.
See, here's the key difference-- SR weak. Because Infernape lacks it, you
have to counter it multiple times, because it
can switch out with relatively little cost. In fact, that's one of the beautiful things about Infernape, playing him as a wall breaker that also can act like a more flexible choice pokemon, able to punch holes rivaling a Specs/Band poke (and potential U-Turn abuse), and be fairly free to switch out and back in again!
Salamence might be difficult to counter once, but then you really only have to counter it once-- or if mence gets forced out, the cost to it is much greater than for infernape.
Physical mix-nape can (will be) perfectly happy to Close Combat Tentacruel/Vappy to the face and switch out to do it again later all day. Try having mence do the same thing. Lets look at your list of Infernape counters, shall we (because some are pretty ridiculous):
Tentacruel: Takes 36-42% from Close Combat, 40-47% from Stone Edge coming from physicalmix Infernape. That's not even taking into account the possibility of a Swords Dance. Factoring in Stealth Rock, Infernape has a chance to kill Tentacruel outright. Even without going immediately for the kill, Nape can switch out, and guaranteed kill tentacruel the next time it tries to do that, without even touching stone edge, just by spamming Close Combat. And with no SR weak, Infernape is perfectly free to do this. Keep in mind that the same Tentacruel takes 20-24% from Overheat, so even if it switches into the most ideal move, 1 SR + 1 Close Combat (first time it switches in) + SR + Overheat + Close Combat (second time it switches in) = a dead tentacruel (even with minimum damage).
Vaporeon: 47.8% - 56.5% from Close Combat from Physical Mixnape, and if I were using physical, close combat would be my spamming move of choice. Assuming SR and Sand (to stop Vappy's Lefties recovery), Vaporeon is not countering this-- period.
Suicune: 53% - 62.9% from Special Nape's Grass Knot on Crocune, Suicune's bulkiest set. And now you know why Suicune does not like switching in directly into Infernape. Keeping in mind that the same Suicune takes 30% - 35.6% from special Infernape's Close Combat and 24-28% from its Fire Blast, so factor in Stealth Rock, and Suicune will be having a hard time countering Special Mixnape ONCE let alone twice. Physical does 35.6% - 41.8% with Close Combat, so unless you're using Rest, you are only countering physical mixnape once too (though you can guaranteed do it once, if you are sure the nape in question isn't a special one carrying grass knot).
Zapdos: Wait, what?? Zappy is taking 62.1% - 73.6% from Special Infernape's Fire Blast. Let's not go into Physical's Stone Edge. Factor in SR . . . no . . . Zappy has no business trying to counter Infernape.
Gyarados: Bulky Dancer takes 29.2% - 34.3% from Physcial Mixnape's Overheat. It takes 47.6% - 56.2% from -1 Stone Edge. Factor in SR, Gyara is dead. Even if it switches in on Close Combat, there's still a chance it could get killed. If you are using a different set, you are dead-- period.
Salamence/Dragonite: LOL, so you are going to switch in, take a hit from STAB Fire Blast/Overheat/Close Combat, take 25% from SR, and then try to survive an HP Ice or Stone Edge (nape's faster)? No f'ing way . . .
few UU pokemon: I think bringing Cress/Regi into Mence's discussion is irrelevant, so we won't bother talking about some pokemon who are probably even less relevant.
Starmie: I left this until the end because it's a bit of a special case (faster and has recover). Starmie has serious set-syndrom, as it cannot figure out whether it wants to be powerful or bulky, and people try to use it to check too many things. As a result, its strength to stop something like Infernape is also weakened. In any case, Starmie can find itself taking significant damage from Infernape depending on what it switches into. Grass Knot from the special set does 75.9% - 89.5%, so even a small amount of risidual damage could mean Starmie's demise. If it takes the time to recover, it leaves itself at the mercy of pokemon like ScarfTyranitar and Scizor who can take the chance to switch in. Ultimately, Starmie's weaknesses to Pursuit and U-Turn from Infernapes Team (or even Infernape's U-Turn), make it only a temporary wall to infernape at the VERY best. But, even with all those issues as an Infernape counter, Starmie is probably still the most solid Infernape counter . . . which I think speaks more for Infernape's awesomeness than Starmie.
Nothing on this list can really counter infernape multiple times, and because infernape lacks SR weak (and resists pursuit/u-turn for that matter), you pretty much HAVE to counter it multiple times a battle.
Before anyone bitches about me talking about multiple sets, Mixnape physical and special are both standard sets, and are more similar to each other than Mence's DD and Mixed sets that people keep bitching about being unpredictable. So yeah, don't go there.
Sorry this post is mostly about infernape, but the list of checks mentioned to nape should seriously be examined if you really think nape has solid counters and want to use that for argument.
ULTIMATELY-- a pokemon doesn't need solid counters to be OU. That's the bottom line, and the argument the pro-Uber side will have to find its way around.
