To be honest, it's not really a big deal. Quick comparison calc:
I honestly think you're overplaying the presence of burn in the OU tier. What uses burn? Rotom-W, which isn't really that common (don't quote usage stats, I mean in terms of usage in higher level play/among better players). Mega Charizard X, there's one, but that's not even its most common set. Heatran, occasionally and specifically for Mawile? Talonflame, occasionally? Burn isn't even that common in the OU tier. How much does it really care about paralysis? I don't think being paralyzed is /that/ big of a deal to a Pokemon that's already slow and has extremely strong priority. You're overplaying the effect of status on a Pokemon that's immune to Toxic whilst burn isn't really that common and it doesn't really care about paralysis. Mega Mawile also barely needs the setup turn. It gets a kill pretty much every single time it comes in against an offensive team, and does severe damage to anything on stall or balance. idk how that requires a turn of setup, it's not like Mega Gyarados or something where to actually bust enormous holes in things it typically needs a DD: Mega Mawile can start dealing enormous and ridiculously threatening amounts of damage right off the bat.
Mawile's speed is a serious detriment and a very good reason to need that set-up turn. Mega Mawile is so slow that on top of offensive checks, there are more than a couple walls/defensive Pokemon that can outpace it, in some cases without investment, meaning not even Sucker Punch can save it from being burned in a lot of cases. And then there's the issue of phasing if it's a SD set
- Maybe it's not USED much, but if Rotom-W is A viability, it should probably be considered to some degree
- Mega Charizard X may not always use Bulky WoW sets, but consider that it's a stall set, the play style Mawile's usually best at murdering. Plus, given its coverage, I could imagine a rare offensive set choosing to run WoW and leaving EQ targets to teammates
- Heatran is beaten by the SubPunch set, which falls into the "really wants to set up" point I mentioned. If Mawile is brought in on a double switch, or the opponent knows to U-Turn/Volt Switch, Mawile's not getting that Sub
- If Talonflame's not carrying Will-o-Wisp, it's carrying U-Turn, which means a check can get in before Mawile sets up.
Mawile's priority is certainly strong, but it's not entirely reliable. Requiring an enemy to attack with slower/non-priority of its own means faster opponents can boost, heal, sub, or status if Mawile mispredicts. Anything that can take a hit switching in and outspends can pull a Non-attacking move to stop it, even if it's as frail as Gengar.
While M-Mawile is already slow, paralysis means that it can't outpace ANYTHING, not even Ferrothorn or Quagsire. The former could Leech Seed it before dying, which is a problem when it has no recovery, while the latter can manage a 2HKO with Earthquake on SD sets, or boost its defenses to stall out SubPunch sets. Quagsire is relevant, so while paralysis won't impact Mega Mawile as much as other Pokemon, it still does allow another common Stall Mon to deal with it. And that's assuming no hax from the status, because while Mawile keeps constant offensive pressure, that one free turn is about all some of its checks/counters need to completely turn the match.
Status is indeed something Mawile has to have a way to mitigate, if not itself then through a teammate.
My point was that while Mawile presents a threat, it needs that first turn to ensure it's not stopped prematurely, whether that's using a Sub to avoid status, or SD to make sure it can break walls. Unboosted Fire Fang has a 14% chance to 2HKO 252/232+ Skarmory with Leftovers, which allows it to either Counter, or Whirlwind. Thundurus doesn't need a Sub or absolutely need a boost to use Prankster Thunder Waves and cripple set-up sweepers, for example.
While certainly among the top for A+,
I feel like Mawile has a decent number of common answers that can prey on its flaws to stop it easily. For A+ Pokemon,
I view those flaws as several small or a few noticeable "chinks/openings in the armor" (metaphorical in the case of offensive mons), while for S Pokemon, they're weaker but nonetheless armored parts of their build, or simply a few small "chinks."