+1 for reasons I'll go into.
Also +1 for the content this thread provides
Disclaimer: I am fully aware PRO is not like PS/PO.
First of all, I think agreeing upon a definition of broken is a good starting point.
Why not look at Smogon's Tiering Policy for this.
To start explaining why I believe Mega-Mawile fits under this description, we have to begin by looking at what makes it broken strong.
First of all, I think the most obvious thing is the outrageous atk stat when taking Huge Power into a count.
It hitting 678 without an SD should make it very clear that it's a powerhouse.
Pair that with the fact it has a priority move, good defensive typing allowing it to come in on a variety of different things and a wide movepool to choose from and you have one of the most offensive threats possible.
Alright so it being obvious that it is a stallbreaker we can look at how Mega-Mawile matches up against the different playstyles in the metagame.
Let's start by judging what it's meant for,
STALL MATCHUP:
Generic stall teams don't really have many answers to Mega-Mawile, their best best would be burning it; looking at possible Wisp users that are playable on stall we're pretty much left with Mega-Sableye & Zard-X.
Here are the calcs vs those mons after a single swords dance, which can easily be done against the other members of the team.
VS Mega-Sableye
+2 252+ Atk Huge Power burned Mawile-Mega Play Rough vs. 248 HP / 252+ Def Sableye-Mega: 342-403 (112.8 - 133%) -- guaranteed OHKO
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VS Mega-Charizard X
+2 252+ Atk Huge Power burned Mawile-Mega Play Rough vs. 248 HP / 100+ Def Charizard-Mega-X: 210-248 (58.4 - 69%) -- guaranteed 2HKO
This goes to show that neither one of these is really a viable option considering Mega-Mawile teams are able to run a cleric to get rid of status afterwards & even if they don't, stall losing a valuable member on their team usual y results in weaknesses being created which once again can be exploited by Mega-Mawile and it's team members.
VS OFFENCE:
The fact Mawile has a good defensive typing and also has access to intimidate pre mega-evolve will mean that it can find openings to come in on a lot of things, upon entering the battlefield in such a scenario, it often forces either a kill or switch. Now, there simply aren't many pokémons ran on offence that want to switch into this 678 attack monster.
Another thing being that Mega-Mawile has the option to run Substitute + has access to priority in the form of Sucker Punch, all you generally need is a bit of prior damage on the opposing team and Mega-Mawile is able to clean up, which should be achievable considering you have 5 other pokémons supporting it.
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I can honestly go on and delve into more specific scenarios where Mega-Mawile poses a problem, but to round things up and not not make my post too lengthy I would like to summarize why I think Mega-Mawile is unhealthy for the metagame and should thus be banned:
1) Unlike most Stallbreakers, Mega-Mawile also poses a massive threat to any Offensive or Balance team.
2) The wide movepool allows for great coverage and has the ability to force your opponent into unpleasant 50/50s.
Eg. Switching is risky because of the Sub / SD Option + Not to begin with the Sucker Punch VS Play Rough mindgames.
3) Whereas a lot of offensive threats / mega's allow for a variety of support teambuilding wise, the possibilities with Mega-Mawile are insane.
eg. Screens / Veil - Sticky Web / Hazard Offense - Trick Room, etc;
4) The pokémon that are able to truely counter Mega-Mawile aren't relevant in the meta in any other scenario thus creating overall weaker teams because you need to check a certain mon, this also limits the options teambuilding wise as those other mons usualy need some sort of support in order to not be useless in a match where the opponent is not running Mega-Mawile
Most of these things like the 50/50 part, the how irrelevant the counters are in the meta and how it limits teambuilding to me atleast showcase that there is a lot inherently wrong with it being in the CURRENT PRO Meta and make me believe it can classify under the previously established definition of "Broken"
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++ As mentioned by the OP, he summarized the similarities between Mega-Mawile and Aegislash, now I'd like to add another comparison to a previously banned pokémon as well. This being Landorus-I with Sheer Force, this was another Wall breaker that did end up getting banned, I personally feel like the counters to that pokémon were atleast a bit more practical / generally viable like SpDef Gliscor.
Anyways, A lot of people seem to want Mega-Mawile around for the sole purpose of it beating stall, I suppose we'll see how these opinions might change the more they face it even while playing Offence/Balance.
Thanks for reading, have a nice remainder of your day.