I've always thought that 4MSS was a pretty poor argument against this things ban on the basis that it isn't Greninja v.s. the entire opposing team. What you have is Greninja+5 teammates versus the opposing team. You cover what Greninja doesn't, and you attempt to overload its checks by pairing it with things that share said checks (offensive synergy). It is basically the entire way that offense functions, and it is why elecspam and darkspam (and, in XY, birdspam) got so popular.
What sets Greninja apart from other offensive Pokémon is a: STAB everything and b: absolutely stupid versatility. Back in XY, there were a number of global answers for Greninja (it actively chose between getting walled by Ferrothorn, getting walled by Mega Venusaur and getting walled by [insert fat water here] and it had basically one viable set (special attacker; I guess it had Spikes too but quite frankly Spikes Greninja both was-and still is--complete and utter garbage). However, with the dawn of ORAS it got Gunk Shot, Low Kick and Ice Punch: three moves which opened up so many options for it. You still have the classically-flawed SpA set, but now you also have mixed attacker and physical attacker on top of this. This allows it to choose exactly what checks/counters it and makes it the single most splashable Pokémon
in the game as a result. Similarly, it literally
chooses what it helps overload based on its teammates.
This allows it to have immense utility as well as near-uncounterability (the only OU-viable counters we have are Chansey, Alomomola and Porygon2: all of which are really funning passive and the latter of which is outclassed by things like Chansey, M-Latias, Clefable, [insert other BoltBeam users here] etc. for the most part. While I admit that Chansey is more splashable than it was this time last month due to favorable meta shifts and people realising that it has actual utility over Clefable on non-stall teams (I've seen an increase in Chansey Offense on the OU ladder, and I've seen it plugged onto more and more balance squads as of late)--although I think it is in part thanks to the discovery of CM+BoltBeam Blissey after the Shadow Tag ban on Smogon giving said Pokémon a niche on stall for its status as the only true counter that stall has to all Manaphy variants (even the almighty Chansey falls to TG+RD Manaphy) in conjunction with its poor synergy for Chansey for very obvious reasons--it doesn't change the fact that literally the only global answers to this thing are passive as f**k, and it doesn't change that the situational answers are way too easy to overload due to the sheer amount of variation that you can have within teams allowing you to splash this with basically any offensive partner that can help it overload shit.
When compared to other overload cores/archetypes such as darkspam, birdspam, keldeo+breloom/medicham etc., who simply hit targets with similar moves and are reliant on lure moves like Hidden Power Bug/Rock Tomb (Keld+Breloom/Medi), Grass Knot/Aerial Ace (Bisharp+Weavile), Natural Gift/Stone Edge (Pinsir+Talon) to not be hard-countered by certain Pokémon, as it simply has no one standard moveset, meaning that every attack it makes before its entire movset has been scouted out is a lure. This is not something that any one Pokémon should be capable of doing without the aid of an ability like Illusion (which actively conseals its identity, turning every time you bring it in into a case of luring), and as such it is problematic. Its a lot like Hoopa-U in this regard in that Hoopa-U forces a 50:50 every game between its Choice Band and Choice Specs sets, forcing the opponent to either make a hard, luck-based prediction that is the difference between losing a Pokémon and not losing a Pokémon (obviously this is risky can very easily screw you over later on, and it is the only way that you can get out of a first matchup against Hoopa without losing a 'mon) or for them to sack something off to scout out band v.s. specs (less risky as you can assess what you no longer need, but it also puts you at an instant 1-mon disadvantage). However, unlike Hoopa-U, Greninja can achieve this
up to four times every game for its entire moveset. Wheras with Hoopa-U you know what its relatively safe switch-ins are from the moment you see which attacking stat it hits off of, you cannot know for Greninja until the moment you see that fourth move, and this is what makes it so problematic. Until you know which four of Gunk Shot, Ice Beam, Ice Punch, Grass Knot, Low Kick, Hidden Power Fire, Extrasensory, Hydro Pump and Dark Pulse its running, you cannot know what answers it. Repeating myself here, but this should only be possible with the aid of identity hiding. Greninja does not have this, and yet it still achives this.
tl;dr I think Greninja is borked.
I feel as is it should not be banned. I think even with its versatile movepool its defenses are made of paper and hazards destroy it slowly especially if its a u-turn set. That being said I think it shouldn't be banned and if we were to maybe ban protein. It maybe fast and can run very many sets including utility but removing protein gets rid of the stab boosts which makes it weaker.
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Please don't use Greninja's defenses as justification for why it shouldn't be banned. It is really funning terrible justification. VoltTurn support and double switching both exist, and we're not suspecting it on its ability to take hits here, and if it were valid reasoning then Skymin (one of the most blatantly broken Pokémon in the game) would currently be unbanned. Also, banning Protean does not lead to a desirable meta because the only thing that is broken with it is Greninja. Froakie, Frogadier and Kecleon are not broken with it. We also want to try and avoid another chicken clause (which I was, and still am, firmly against) if we can due to it adding unnecessary complexity to the tier list.