No ban.
We know it has really good stats. We know it has coverage moves for almost everything. We know it's bulky. So stop making that your sole arguments.
Mega Metagross has tons of soft checks. Yeah, if Slowbro tries to come in it risks a 2HKO from Grass Knot. Mega Sableye tries to come in it risks a 2HKO from Meteor Mash. Jirachi risks EQ. Rotom-W might take Zen Headbutt. Mega Man might take Zen Headbutt or EQ. Hippowdon might take a Grass Knot. Gliscor might take an Ice Punch. Tankchomp might take an Ice Punch.
But what do all these pokemon have in common? Besides the one move they fear, they all are, at most, 3HKOed by the other 3/4 of Mega Metagross' moves. And these are a diverse set of pokemon (we've got Water/Psychic, Ghost/Dark, Steel/Psychic, Water/Electric, Electric, Ground, Ground/Flying, Dragon/Ground, and I'm not implying these are the only ones) and many have reliable recovery; it's easy to, even unintentionally, stack them. Which leads to the next point:
Mega Metagross is prediction-heavy. I know prediction is an "unacceptable" argument but an exception should be made here. Let's say Mega Metagross is up against a team that has three soft-checks to Mega Metagross (which isn't unreasonable or such a strain on teambuilding, as seen above,) and it's up against Clefable. By having these three checks, is the opponent really going to lose a pokemon here? They probably won't, actually. With Greninja, its speed and specificity of its checks, this wasn't a problem. It was hard to stack checks. This isn't the case with Metagross; yeah maybe they'll use Grass Knot predicting I bring in Slowbro, but I can just as easily bring in Jirachi or Tankchomp. The odds are against the Metagross user.
Mega Metagross' moveset is also stricter than Greninja's was. Greninja could run what fucking ever cause everything was STAB. You can't run Ice Punch and Grass Knot at the same time, or else steels walls you. Meaning bulky water+Gliscor/Garchomp/LandoT is enough to handle pretty much every set.
Another problem with its moves is that Hammer Arm lowers its speed. It's something that's at the very least worth mentioning. And while you can beat Ferro 1v1 with it, you take a lot of damage in the process. Iron Barbs+Leech Seed+Leech Seed (Protect) and then another Iron Barbs at minimum, assuming something doesn't switch in to the Hammer Arm and force your slow -2 ass out (and take health through Seed).
Luxury moves only make this problem worse. Agility, Bullet Punch and Pursuit being the main moves you'd
like to run on it but often can't. Don't say "Agiligross is a nightmare for offensive teams!" when dropping Grass Knot means a lot of bulky waters now check it.
We don't ban things for having no counters. See
this post.
But it has counters anyway. Bulky Mega Scizor, very healthy PhD Mew, Alomomola, and Brongzong can all wall it. Counter Skarmory can, uh, counter it. Oh and Arcanine. So Stall teams have answers.
It can't simultaneously check the things you want it to check and be hard to revenge kill.
252 Atk Landorus-T Earthquake vs. 0 HP / 0 Def Mega Metagross: 252-296 (83.7 - 98.3%)
Wow, it avoids an OHKO from even Lando-T! You'd need at least 16.3% damage for a guaranteed KO, or 10% if you count Stealth Rock. Where or where could you find that?
0 SpA Clefable Moonblast vs. 0 HP / 4 SpD Mega Metagross: 45-54 (14.9 - 17.9%)
Oh.
Another example: Mega Gardevoir Hyper Voice: (34.5 - 40.8%). After Garde's Hyper Voice, now you can't survive Thundurus' Thunderbolt (57.1 - 67.4%).
Latios' Draco Meteor does 41.8 - 49.5%. That's also a pretty big chunk of damage, against something with no form of recovery. Mega Lopunny can now revenge you relatively safely (51.1 - 60.7%). Special Mega Altaria does 28.2 - 33.5% with HVoice, which puts it in range of Bisharp's Sucker Punch (72.4 - 86.3%).
In actual conditions, Mega Metagross can't tank hits forever. Especially when you consider that it starts with base 70 speed; it's probably going to take a hit on its MEvo turn. Of course, there are revenge killers that need little to no prior damage to revenge kill, some examples being Scarf Heatran, Mega Manectric, Talonflame, Victini (this takes a dump on all sets that don't run EQ btw), Mega Charizard X, Bisharp (w/ Dread Plate), and Mega Houndoom
maybe we should ban Stealth Rock so fire types become more viable. Excadrill under sand and Mega Swampert, Kingdra and Omastar under Rain are also able to outspeed and OHKO.
Hax goes both ways js cause like every move it has is 90% acc, don't cry about 20% chance for boost
So tl;dr- REALLY good mon, I agree, but it's not as unstoppable as people are making it out to be.
-Easy to stack checks against it. Checks to it are available throughout the meta, none are specific
-It has counters that one can choose to use, some of them not being specialized counters
-There are things that RK it from full, and it can be 2HKOed by a large number of combinations
Also let it be known that Float Stone Slowbro counters Mega Metagross (◕‿◕✿) w/o thunder punch