I'm not entirely sold on the idea that this thing is that broken. One of the main arguments I've seen against Gren so far is that it puts too much pressure on offense, but I don't think this idea holds much water seeing as how it gets slapped around hard by most of the prominent forms of priority and that because it's forced to run Life Orb it can be revenged by scarfers quite easily. Plus, while it's very fast, it's not THE fastest threat - we got three new megas that all outrun and OHKO it in Lop, Bee and Sceptile, in addition to the two we already had in Manectric and Aerodactyl - all five of which are good, viable pokemon in this tier.
Similarly I've seen people say that Chansey and Alomomola are the only counters for defensive playstyles, especially stall, but to be perfectly honest I don't think it's unreasonable to expect to have to run at least one of these if you're using a stall team simply because both of these are such damn good members of a stall team, and to be honest, if you're running stall your teambuilding options are already quite badly restricted anyway.
That's my overall take on the issue, that yes, while Greninja is clearly an amazing Pokémon with its incredible speed and great mixed power thanks to its multistab ability, answers exist for it in OU already - not nonviable stuff that you have to dredge up from a lower tier that doesn't work against the rest of OU, stuff that's good at fighting the rest of the metagame, not just Greninja. There's a multitude of good checks and even a couple of good counters.
In addition, while Greninja's positives are obvious, I think its negatives must also be taken into consideration. The most obvious one is that this thing is frail in every sense of the word. While very little switches into it safely, Greninja has arguably even less opportunities to safely switch in itself. Add to the fact that Life Orb use is pretty much mandatory and that Greninja takes a full share of damage from spikes and SR and that its uncomfortable with every form of status that exists and that its base water / dark typing is very poor defensively and that because it goes mixed most of the time it's forced to compromise its defenses even more and that Protean can be an achilles heel in that it can open up fresh weaknesses for an opposing pokemon to exploit, its lifespan is a lot more limited than you might think. There are two ways to get this thing in unaccosted to do its job - pivot it in with U-turn or Volt Switch, or bring it in after something of yours has fainted. Plus, while I get that Greninja does have many tools to beat many different pokemon, it simply cannot run them all at once. While I get that you're not able to 100% know ahead of time because of the multitude of sets it can run, I feel like most of the time, if you're a good player (and I concede that I am not) you can take a look at the rest of your opponent's team and at worst be able to make an educated guess about the kind of coverage it's running and the threats that it's been included to cover.
So, yeah. While this thing is obviously very good, the question is whether it's too good for OU. On balance, I don't think it is. While I feel like it does restrict teambuilding somewhat, it's not the only pokémon with good attacking stats and coverage that does so. In addition, I don't feel like every option for beating this thing has been explored fully yet. While I'm not saying that I've got all the answers, I feel like people should be considering things like good users of the Assault Vest item to be able to take its hits easier, more use of choice scarf and strong priority to force it out, Protect to scout its moveset, etc. etc. While I can see eye-to-eye with the pro-ban point of view, and understand if it does get banned, my own point of view is no ban.
PS.
Of course, I can see already that there's a lot of very understandable pro-ban sentiment, and I'm pretty sure that I'm gonna be on the losing side when all's said and done. I can also tell, sadly, that a lot of the anti-ban posts are gonna be bad posts using poor arguments, basically grasping at straws to save their favourite pokémon, which won't help the anti-ban faction any. I can understand this, I love Greninja as well and I'd rather it didn't go to ubers, that's why I care enough to make a tl;dr post instead of just shrugging and ignoring the thread. If you're going to make an anti-ban argument, please don't make a low effort shitpost that betrays you as an inexperienced player who just doesn't want to see their favourite poke get flushed. Take some time to understand the meta and back up your points with reasoned points, it'll both help your argument and result in less dead brain cells for everybody. I'm sure this thread will be here for a while.