Well, as a Mega Mawile user, and a battler who has faced mawile many a times in battle, I can safely say that mega mawile is not unhealthy for the metagame and must not be banned from the OU metagame.
First of all, Mega Mawile is not as bulky as people think. Non-mega evolved mawile has pathetic stats in defenses, and struggles to switch in to attacks. Also, even though mega mawile has defenses of 115/95, it is counteracted by it's shitty HP stat of 50, and many other sweeper mega evolutions can stomach attacks better. Also, Mawile has weaknesses to the hugely common fire and ground type attacks, and most teams carry a powerful pokemon with a move of either type. Therefore, it is not so difficult to KO Mawile as some people say. Mawile hates facing ground types and fire types, and even when I battle vs a mega mawile i simply send out a landorus-t, garchomp, or any charizard and smash it with their stabs.
Secondly, Mawile is too slow to be an effective sweeper. It's base speed stat is 50, and for it to counteract it it requires team support like paralysis of the opponent mons or sticky web. People might say that it has sucker punch to make up for the low speed, but they do not realize that despite huge power, mawile is forced to use a non-stab priority move which isn't boosted by an item, and can be exploited by using status moves (switch in rotom-w on an expected sucker punch and burn it), and it's sucker punch without a SD boost is often lacking in power to KO it's biggest checks, such as lando-t, garchomp, etc. This is perhaps Mawile's biggest flaw, and the reason why it is not the perfect sweeper.
Thirdly, Mawile's attack stat is monstrous, but not impossible to wall. Mawile often suffers from 4MSS, and thus can be walled. Most Mawile users run play rough, sucker punch and a setup move as the standard 3 moves, leaving the last slot open. If mawile lacks focus funch, it is completely walled by heatran, who can just block its attacks and spam lava plume. If it lacks fire fang, it is completely walled by skarmory, who can block it's attacks, heal itself with roost, and whirlwind away any attempt mawile makes to setup. If it lacks iron head, most mega venusaur variants can simply wall it, put it to sleep, break it's substitutes with HP fire, heal itself with synthesis, seed mawile, and take pitiful damage from the supposedly monstrous fairy. Mawile has no real way to heal it's HP other than rest, which hardly anyone uses, and the rest set is even easier to counter than most, so Mega Mawile can be easily worn down with repeated damage, entry hazards, etc. Also, mega mawile is prone to burns, as a burn can cut it's attack in half and bring it down to earth, and paralysis can make it even easier to KO mawile. Rotom-W is especially effective at burning it as it can survive play rough on the switch, outspeed it and burn it with WoW, crippling Mawile.
The fourth reason why Mawile is not unhealthy for the metagame is that it can be checked, countered, and crippled by many pokemons commonly used in the OU metagame. It can easily be revenge killed by garchomp, landorus(incarnate and therian, therian's better), charizard forms, keldeo, and others, and it can also be walled/countered by pokemons such as gliscor, heatran, mega venusaur, skarmory, etc, and these aren't even obscurely used mons, some of these guys have topped usage in the recent past.
Mega Mawile is not broken guys, let's be reasonable and see the reality. Personally, I wouldn't mind seeing Mawile depart to ubers, but I firmly believe it is neither invincible nor unhealthy for the standard metagame.