The thing is, why would they need to? I'll tackle these one at a time.
Swampert: Takes about the same amount from MixMence DM as it would from DDMence's +1 Outrage. In both cases, Mence is left crippled, whether that be an unfavorably low Sp.A or by being KOd with Ice Beam. If you want to save Pert as death fodder you can - the lowered SpA will allow the steel of your choice to come in and end Mence's sweep right there.
Suicune: Will never be KOd by DM + Earthquake, which means the only think Mence can do here is Outrage. Bring in Metagross / Jirachi and call it a day.
Hippowdon: You know Salamence isn't going to lock itself into Outrage, so use an Earthquake resist to take it down. Or, just use a pokemon that doesn't take that much damage from an unboosted 310 Atk LO Earthquake (there are plenty). Also note that Hippowdon is prevalent on stall teams, which often have Skarmory or Forretress as a backup, meaning that if you got rocks up, you haven't lost too much.
Celebi: Is rarely OHKOd by any attack, meaning that Mence takes 2 rounds of LO instead of the usual one, and that translates into an easier revenge kill. I get the whole "but then it has done its job" argument but really, if you are trading Celebi for Salamence then it is a fair trade. Not to mention that MixMence often leads off with DM instead of FB (and hardly ever with EQ), so bringing in a steel is often the best move here. Heatran in particular makes a nice switchin, and is often seen alongside Celebi. You'll note that I'm acknowledging that it isn't Mence vs. 1 pokemon, it is Mence vs. a possibility of 6. It has to predict perfectly to score these crucial KOs, while I have to predict what the most likely attack is and switch accordingly aka Mence has to predict Heatran, while Heatran only has to avoid Earthquake.
Gyara and Donphan: Touche. Here, they should only be brought in if Mence is revealed late-game, signaling a DDer. ResTalk Gyara (often found on stall and balance) is often paired with some type of special sponge, which would be the better choice. I'm not saying that you should always switch Blissey into Mence but, given that Mence does not typically select Outrage as its first attack, it may in fact be your best choice against a mid-game Mence.
Shaymin and Slowbro: Shaymin will always survive the ordeal, and can come back in later on a bulky water or ground to Natural Rest the damage. Slowbro is crippled indeed...maybe I should look into Slowking?
Torterra and Regirock: Torterra is 2HKOd, which means 2 rounds of LO for you. Regirock takes relatively little damage from either of the two sets, making it a decent candidate as a primary switchin. Also, Tyranitar is a common partner for Mence, so it is quite likely that sand will be up and running, lessening the damage it takes.
The point holds true that switch-ins differ for each set. I could say the same for Infernape, Jirachi, or Tyranitar.
It is interesting that Salamence is firing off Draco Meteors free of all consequences. Taking Stealth Rock + LO recoil and ending up at -2 is far from an ideal situation, and in some cases it takes two hits to finally KO the opponent. This leaves Mence vulnerable to faster threats, such as ScarfTran or LO Starmie. And since the set up for the support characteristic is Modern Mixmence and not Classic MixMence (which can use Roost to mitigate passive damage), it is not coming in and wreaking havoc again.
EDIT: To keep this from being pure theorymon, I'm just pointing out that I have no qualms with using these pokemon on the suspect ladder. Swampert, for example, has been working out just fine for me, though I've been playing a bit with the older 252 / 212 / 44 SpA spread I used to run and it is giving me some good results. Donphan and Torterra aren't seeing too much use what with all the Shaymin/Starmie running around by my own Celebi works well with the defensive spread, and is great for taking on Breloom and Machamp if I can score a Leech Seed. I find that most of my changes have been to EV spreads rather than actual pokemon.
