Dragon is the best attacking type in the game, hands down. Neutrality to everything sans one type is much more valuable than hurting some things for SE damage. ...
Oh no, it's probably the best
single attack type in the game. It loses its luster in combination with other attack types because Dragon already hits nearly everything unresisted and the second attack doesn't bring a whole lot to the table. Concerning the aforementioned type combos, hardly anything resists those either. (Absolutely nothing for Fighting/Ghost, in fact.) Once you're at that point, type advantages win out over lack of resists because the other type attack covers against any resistors anyway.
Flying/Fighting is also only resisted by two pokémon (Zapdos and Rotom) and has eight type advantages. There may be others I don't care about trying to figure out.
"Garchomp also has a notable resistance to Fire."
Fire is hardly notable, it's basically synonymous with "Heatran" and all of the dragons (and Gyarados) resist it anyway.
"Dragon/Flying is worse typing than Dragon/Ground. Sure you can come in on Fighting attacks, but you are punished every single time you come in even though you have the resistance. This lowers how much you can actually sweep, eliminating most of your usefulness before you even start. On top of that, Flying STAB is worthless whereas Ground STAB (which is SE against the only type that resists its other STAB, mind you) makes Garchomp the potent offensive threat that it is."
It doesn't eliminate Salamence's ability to sweep, switching into SR and taking 10-20% damage, any more than Garchomp is unable to sweep after shrugging off SR but taking 30%+ damage getting in and/or setting up. (Unless you switch into Rock damage, the only notable of which is CBTar.) Flying STAB is useless, but Flying is better defensively and Garchomp only uses its Ground STAB on two or three things anyway. (Metagross and T-Tar come to mind.) It's all about the SD Dragon attack with a side of Fire Fang to cover the likes of Skarmory and Bronzong. :/
"And even then, Garchomp can be extremely unpredictable."
Technically so, yes Garchomp really isn't all that one-dimensional. It's quite rare to see Special off-sets, though, and it's only useful due to the surprise factor. Both physical and special Salamence are dangerous in their own right and it does mixed sets better too, you really have to be on your toes until you know its moveset. Physical isn't even what most people expect when they first see it either.
"Advance usage arguments aren't relevant because they can't be proven and because this isn't Advance. Garchomp usage continues to rise because stopping it cripples your opponent's entire team."
It's the exact same argument. 'Garchomp is being used by an awful lot of people, it must be overpowered!' Pray tell why heavy usage correlates to overpoweredness when it's never been viewed as such before. Who cares that Garchomp is used more than Blissey right now? Great, so are seven other pokémon (in weighted usage, which is what you referenced). Are Gengar, Gyarados, Bronzong, T-Tar, Heatran, and Lucario all overpowered too?
I'd give outside "proof" of Blissey's Advance usage but I assume most of the stats died when the NB forums went down. :/ You'll just have to take my word for it, sorry bub. Blissey topped out around 20% and nearly doubled the next most-used pokémon, Garchomp isn't even near those kind of figures yet as far as I hear. (It would be great if anyone else is able to dredge something to back me up here, or even disprove.)
"And the whole "even though it requires two pokemon to beat, after Garchomp gets its first KO it is at low HP" argument is ignoring a pretty important part of the situation: Garchomp is still alive and one of your teammates is not."
Plenty others also "require" multiple pokémon to take them down. Salamence and Dragonite can pull the same crap with DD that Garchomp can. Lucario can SD too, packs priority, and has opponents in the dark about its moveset. Deoxys-S is nigh-unspeedable and has such an insane movepool that potentially nothing is safe. Consider yourself lucky if you only give up one kill to Metagross or Gengar, they pack suicide moves.
Of course, it's never as simple as "use [these pokémon] for guaranteed victory," even for Garchomp. And what does that attitude say about defensive pokémon? The likes of Blissey, Skarmory, Gliscor are hardly useless despite the fact they typically don't directly kill anything.
"Garchomp doesnt NEED to boost its speed. A +2 Atk boost with 333 Speed is much, much more valuable than only a +1 Atk boost with 405 Speed (270 is standard DDMence IIRC). It normally is a balanced trade-off between DD and SD (example: Rayquaza) but the trade-off between DD Salamence and SD Garchomp is not balanced."
Simply put, I disagree. They are merely weaker to different things. Garchomp will takes hits against stuff like Gengar or Scarfcross, Salamence will take hits from stuff like Metagross (no Ground STAB) and Snorlax (may fail to 2HKO).
All else being equal, I still consider DD the better buff. It allows for lower Speed, freeing up EVs for defenses and possibly trading in the +Speed personality for +Attack. (Dragonite can't hit the 'coveted' 270 without +Speed, but even at 259 the only extra pokémon able to outspeed him are Jolteon and Aerodactyl.) The ratio of power between one SD and one DD is 4/3 (~33%), yet one-SD Jolly Garchomp is less than 20% stronger than one-DD Adamant Salamence. And not only is it easier to DD twice than it is to SD twice, since moving from second attack to first attack is a free turn, but the benefit is greater for doing so. (Two SD is usually overkill anyway, a second DD brings you up to one-SD power and has you outspeeding even the likes of Deoxys-S.)