Ice Shard is very uncommon statistically. I think we can all agree that its main role is to revenge kill Salamence. Why is Ice Shard uncommon is Salamence is so broken? Because its not that broken and people don't need Ice Shard to take it down because half the time, it'll be dead in a few turns from residual damage.
Or maybe it's because the large majority of Ice Shard users are not viable in OU outside of niche roles. Mamoswine is the only one that can even claim to have some significance, as Weavile is crippled by Stealth Rock, weak base power STAB moves, frail defenses, and is much more easy to play around. Donphan's Ice Shard is unSTAB, and other Sharders like Glaceon and Lapras don't make the cut for OU. I may have missed a few, but the same will generally apply to them.
So Salamence is almost guaranteed at least 1 kill per game. The same could be said of Scizor, Tyrannitar and other pursuiters. If you get them in properly and predict correctly, your opponent loses something. Even other pokemon can just kill something because they use a random move on the switch, such as Infernape.
This is untrue because Scizor and Tyranitar's Pursuit will only be KOing particular types as they switch out. Jirachi or Vaporeon certainly won't be KOd by Pursuit unless they are severely weakened, so this really only applies to the likes of Starmie and Latias. Salamence, on the other hand, has its perfect coverage trio of moves in addition to high offensive stats that allow it to inflict crippling damage regardless.
Intimidate is a good ability, no doubt. Salamence isn't the only effective user. Gyarados has similar defensive stats, argueably better defensive typing and the ability to run defensive sets as well. So what if Gyara can't go mixed. Going mixed is mainly for wallbreaking and taunt does it for gyara, while DM and Fire attacks do it for Mence. Scarfers with the same Rock moves will kill both Gyara and Mence, while Scarfers with Electric moves other than Rotom are uncommon compared to those packing Dragon/Ice Moves. Gyarados can't even be picked of easily by bullet punch. We do not consider Gyarados Broken, so why Mence.
Gyara's ability to run defensive sets is limited to ResTalk essentially, a much less efficient method than Roost or Wish, we will all agree. And the thing is that regardless of its set, Gyara is
still countered by several pokemon, those being defensive Rotom-A, HP Electric Vaporeon, and Celebi (bar Bounce). And while a ScarfGon is a reliable revenger for Gyara, it only has a 50% chance against DDMence - the speed makes a difference. To answer your last question, the answer is because they are not the same pokemon.
The whole counter mentality is stupid. If I really wanted, I could slap the appropriate HP on quite a few pokemon with decent special attack and run a lure set to kill my counter.
Another thing is that while more often than not, you'll be forced to revenge kill Salamence because it got that 1 kill, fact is Offensive Salamence isn't the kind of pokemon that you can easily switch into an opponent unless its completely helpless against it. I for one, can't think of a single pokemon that cannot within reason, run 1 move to kill mence either on the switch or will neuter it if the Mence user predicts a switch and uses the wrong move. HP Fire Celebi can T-Wave Mence, Bliss can T-Wave, Hippo can run Ice Fang to kill Mence's staying power etc.
No one here is even arguing counter mentality anymore. We have all generally accepted that checks are what matter this Gen, and there are very few reliable ones to Mence. As far as your other point, you've reduced dealing with Mence to merely anticipating a switch and attempting to cripple. If Tyranitar is out against your TinkerBell Celebi, are you going to risk a Crunch and Thunder Wave, or will you go for the kill and Leaf Storm? Either situation has its pros and cons, and the former choice could be quite costly if they don't switch in Mence like you planned. Ice Fang on Hippo is really only for Mence, anyway.
Salamence is top tier OU. Of course most of the possible sets to run with it have been discovered. It says somewhere on the analysis that Mence is fairly unpredictable. Not true. You can guess from the stage of the game when Mence first appears (i.e. Early/Mid/Late), its partners and usage statistics what your most likely dealing with.
Somewhat true. Assuming the two most common sets (MixMence and DDMence), both can have the same partners and yet be different sets. For example, CBTar would definitely appreciate MixMence crashing through bulky waters and grounds, but DDMence can at the same time benefit from a powerful Pursuit used to remove ScarfLatias or Starmie from the match. Same partners, different strategy. So this is not fully applicable.
Next, Salamence wouldn't be so threatening if people just didn't play so stupidly half the time. Seriously, if you got swept by Salamence, it may be beacause you were reckless. If I see a Salamence, i shouldn't just switch in my Swampert just in case it eats a Draco Meteor or LO Outrage to the face. Salamence just switched in on your Breloom without a sub and you're afraid it will DD and sweep your team. Just Focus Punch it and settle for some damage instead trying to absorb an attack. If its DD Mence and it DDed, congratulations, it now doesn't have enough HP to sweep your team. If it wasn't, all you lost was your Breloom. You may say "But I needed that Breloom to win!" Well, assuming a +1 DD Mence would sweep your team anyway, at least you have hax to rely on instead just being killed off.
So Mence is threatening because people play stupidly? Okay. So what do you do when that DDMence turns out to be of the bulky variety, and a switch to Swampert could have actually been the right choice? And in your scenario, you are still losing a pokemon. DDMence boosted up, killed your Breloom with Fire Blast, and is free to attack another round. If not, MixMence removed your Breloom and is still free to attack another round. You were still forced to sacrifice your pokemon, so Mence has done its job in the end.
Yes, Salamence can do all these wonderful things, like be a wall breaker, set up and sweep you, be surprisingly tanky, wish support, whatever, but it can't do all of them at once. Once again, paraphrasing the analysis, Mence is a lot easier to handle once you know what you're dealing with. If you're desperate to kill Salamence, keep a Steel Handy and force it to Outrage and/or waste its DM. Pokemon that do this aren't hard to come by. Gyarados, Flygon, even other Salamence.
The thing is...it can do all of these things equally as well, which few other pokemon can boast. Sure Gengar can run a LO set and a SubPunch set, but will the latter be sweeping as well as the former? And, if you can use the rest of your team to lure an Outrage, Mence player can lure out your steels to trap and kill them with Magnezone. And then, it could always run Dragon Claw to foil that strategy and possibly even get a second boost.
Bottom line, Mence is not broken, its just people don't know how to play around it.
Because they can't do so without resorting to a stringent list of reliable checks, the large majority of which are Pursuit bait.