Also from experience alone I know you'll never get off that Scyther Swords Dance without being hit at least once. No person is stupid enough not to attempt to cripple Scyther while they still have a chance. In fact LO hurts Scyther alot as during its setup chances are it'll be hanging onto the edge of its life more than often. Thats just how its defenses work, they usually allow it to survive one good strong hit but nothing more.
It's not quite that simple. If they do try and attack, Scyther can just finish one of their pokemon. And the person playing Scyther is usually going to aim to come in on something that isn't a huge threat to it.
In fact heres the major problem with this whole situation. Your perfect scenario of SD'd Scyther w/LO assumes your opponent is a borderline idiot incapable of any sane decisions. One who doesn't realise Scyther is suspectible to every attack in the game not Fighting, Ground, Dark and Bug. Which is one of its inherant weaknesses, Scyther can be hurt by just about anything and sometimes its better not to switch.
Scyther's nowhere near as much of a glass cannon as you're making it out to be. Tank of the year it isn't, but there are plenty of pokemon that Scyther can come in on and SD against without facing an overwhelming amount of damage.
Also, Stealth Rock isn't something that's always going to be up. There are still decent spinners in UU (I like Hitmontop, myself), and you'd be an idiot to not have one if you're using Scyther. For the record, though...Drifblim will always be 1HKO'd by a SD'd LO AA if Stealth Rock is up.
Hyra, I agree with your statement up to this point:
Its Baton Pass is comparable to the appearance of Scarf and Chain Chomp.
Baton pass gives a lot more versatility and offensive options to Scyther than Garchomp gets by running a slightly different moveset (well, and there's the fact that Cresselia has reliable recovery, unlike many UU walls).
Actually (and I'm not using this as an argument, I just felt like mentioning it), there's one other point as well - all of the pokemon I did calculations for sans Shuckle (and Quagsire, who has a chance to live with something like .25% health) would be KO'd by Scyther's attacks if there was SR and 3 layers of spikes. Admittedly, that's a huge amount of setup and it's highly unlikely it would ever happen, but it's certainly something Garchomp can't do.
Edit: Golem can survive, too.