I would like to note to readers that the term 4MSS should not be used for mons with just a very wide coverage that cant pick more than 4 of their multiple choices, rather than for mons that need certain moves to be useful and then having to choose between some coverage moves, even poor ones, with the risk of getting walled by certain main threats.
Let's go with an example
BW Dragon Dance Dragonite is the clear example of 4MSS mon; it needs Dragon Dance to sweep, Outrage as STAB, then it has to choose between Fire Punch to hit steels, specifically Skarmory, Scizor and Forretress; Earthquake for the same reason, hitting specifically Heatran and Tyranitar; Extreme Speed to not get revengekilled by Mamoswine and scarf users like Keldeo or Latwins. Lacking even one of those move makes Dragonite weak to certain top metagame threats, therefore it clearly suffers 4MSS.
Protean Greninja is the clear wrong example of 4MSS mon; it can run Hydro Pump, Gunk Shot, HP Fire, Ice Beam, Dark Pulse, Extrasensory, Low Kick, Spikes, Grass Knot and many other moves, but giving up on one of these moves doesn't make Greninja incredibly weak to certain top threats. Sure, you would like to run HP Fire when facing Ferrothorn, or Gunk Shot while facing Tapu Fini, or Extrasensory when against Toxapex, but Greninja's job is just to spread damages and lack of a specific move doesn't block it from doing that. For this reason Greninja doesn't suffer 4MSS.
Mega Metagross clearly belongs to the second category. It doesn't actually need to run all of its moves to be effective, it just has to spread damages (mainly with Meteor Mash) to be dangerous; running Hammer Arm over Earthquake will make u stronger over Ferrothorn and weaker vs other Metagross but that doesn't stop the Pokémon spreading damages and being dangerous, since it doesn't need any boost move to be effective, nor its whole coverage gets walled by top threats unless running that specific move for them.
I feel like Metagross has an important defensive sinergy in offensive teams, being very useful against many psychic/fairy/flying/ice pokemon, as Tapu Lele, Mega Pinsir, Kyurem-Black, Tapu Bulu, and that's the main reason it's so used; like Landorus-T, another top metagame mon, it makes the difference giving actual switchins on other non-defensive mons, letting your offensive team not being a "sack party" when in passive situations. For this reason I believe Mega Metagross has important use for the metagame, and I didn't appreciate the choice of not suspecting some other dumber mons before it (*ribbit*)
However, due to things you would get tired to read, as gen 7 mega evo buff, powerful coverage moves with a lot of secondary effects, the same coverage moves able to potentially cover every threat, the very little cost in playing a mon that gives a lot of pressure and makes opponents hope that its coverage can't destroy all of their mons, I believe it should still be banned. Maybe it's not the right timing, but it's not actually worth waiting more time and keeping this beast in the tier just because the future could make it less dangerous, there are too many reason to let it go opposed as reasons to make it stay, imo.