I would like to point out that if having no counters was a sufficient argument for a ban, then Gothitelle and Dugtrio would be banned and Wobba would still be uber because by the definition of "able to switch into and scare out" that's virtually impossible. I would additionally like to point out that technically speaking Kee Berry Mandibuzz can switch in on PuP and threaten to OHKO with Foul Play, and indeed anything faster with STAB Foul Play that doesn't die to Sucker Punch (given that it'll resist due to Dark typing, that's almost all of them, even Zoroark can cop the Sucker Punch and OHKO back) can force it out on revenge, not just Keldeo et al (Not that this particularly makes it balanced either). Additionally:
+2 252+ Atk Kangaskhan Sucker Punch vs. 4 HP / 0 Def Pinsir: 125-148 (88.6 - 104.9%) -- 31.3% chance to OHKO - Without the +nature, it fails to OHKO and with any defense investment from Pinsir it also fails to OHKO. So Mega Pinsir potentially has a shot (besides which it can foil Sucker Punch with a Quick Attack off a superior Speed stat, easily OHKOs with a strong Fighting move, and takes a pittance from PuP)
Additionally, Gengar can switch in on anything that isn't Crunch (and if they do use Crunch you can just chuck in Lucario for free) and force it out or trade with it directly via Destiny Bond. Anything faster that can phaze and has Focus Blast (Noivern is a good example) can come in on PuP, take minimal damage, and then phaze it out on Sucker Punch or destroy it on any other move.
Finally, Full Defense Aggron can also handle Mega Kanga well with minimum prediction. It takes an absolute pittance from Return, especially pre-mega:
+2 252+ Atk Kangaskhan Return vs. 252 HP / 252+ Def Aggron: 33-39 (18.6 - 22%) -- possible 5HKO
+2 252+ Atk Kangaskhan Return vs. 252 HP / 252+ Def Mega Aggron: 54-64 (30.5 - 36.1%) -- 51% chance to 3HKO
And after Mega Evolving soaks Power-up Punch with equal ease:
+2 252+ Atk Kangaskhan Power-Up Punch vs. 252 HP / 252+ Def Filter Aggron: 43-52 (24.2 - 29.3%) -- 99.9% chance to 4HKO
Unfortunately Kangaskhan is JUST heavy enough to avoid the 100 damage from Heavy Slam, but you can easily phaze it out with Roar or simply go for the kill:
4 Atk Aggron Brick Break vs. 4 HP / 0 Def Kangaskhan: 78-92 (43 - 50.8%) -- 2% chance to 2HKO
While you can't get much better than 2HKO with SR, Even +4 PuP is not a threat, especially when Aggron came in resisting SR:
+4 252+ Atk Kangaskhan Power-Up Punch vs. 252 HP / 252+ Def Mega Aggron: 64-76 (36.1 - 42.9%) -- guaranteed 3HKO
Two uses of Metal Burst are even an option, unless I'm badly misreading the total damage from that would be ((43 ~ 52) + (64 ~ 76)) * 1.5 which comes up at a minimum of around 160, with Mega Kanga's 4 EV HP being 181 this is very likely to KO even without SR (and certain with it).
Additionally, it's possible to manipulate the fact that PuP is very obviously the best move available thanks to the boost it gives to bring in something else for free (Gengar, say). The only thing Aggron really has to fear is coming in unevolved on Power-up Punch, which while it still has the 4x weakness will obviously mangle it horribly. Fire Punch/Drain Punch variants also have some slightly more favourable outcomes, but lacking Crunch leaves it wide open for Ghost stall.
That said, the fact that there aren't any HARD counters does say something about the situation. The fact is, you can throw out Mega Kanga without too much thought (on revenge if nothing else) and your opponent HAS to rely on prediction because Mega Kanga almost certainly has a winning move against every Pokemon you've got. While I stand by my opinion that there are way more Pokemon that can beat Mega Kanga with decent prediction, I'm reasonably confident from all the arguments made here that there are none that can beat it without any prediction, which means that even assuming perfect prediction you need two teamslots to deal with the two different predictive scenarios. Given the uncertainty you probably need three in case you mispredict or they outpredict or even you just need one of the anti-Kanga brigade to deal with something else.
To sum up, there are more soft answers (checks, I guess, but not JUST checks) than people think (Musketeers, Infernape, Mienshao, Mega Luke, Scarf Breloom, Scarf Luke (lol), Gengar, Noivern, Zoroark, Liepard (also lol), Weavile, Sableye, Mandibuzz (but only just), Volcarona (dodgy luck-wise, though), Rocky Helmet Ferrothorn, Rocky Helmet Garchomp, Rocky Helmet Skarmory, Cofagrigus, Froslass (is very weak but can guarantee a hit at least, bringing him into priority range), Aggron, etc), but still not enough to make up for the lack of hard answers since you need multiple soft answers in order to prevent it from mangling your team, even accounting for the fact that predicted Sucker Punches are a free switch-in. As such, I think I'll throw my support behind a quickban since I wouldn't want to have to build a team with three of these Pokemon on it in order to succeed.