The inevitable question with Onix vs. Scyther is STAB Stone Edge vs. STAB Aerial Ace.
...What Scizor uses Aerial Ace? =\
Besides, 70 base Speed and an unfortunate 4x weakness to Water slaughters its chances of standing out, especially when you consider that Steelix is UU itself, and UU loves to rain on our parade. (Aren't I just sooo witty? Har har friggin' har. :p)
70 base Speed in a tier littered with Swift Swim doesn't help its inability to "sweep" (we also must compare 45 base Attack to 110. And what is this, really?)
Scyther, on the other hand, has plenty good stats to stand out on its own.
But if you want to call out Scyther and Steelix for being "alternate" evolutions, Slowking, Politoed, Kingdra, Electivire, Magmortar, Rhyperior, Porygon2 (and Porygon-Z), and many others that I'm forgetting, would like to have a word with you.
...Gah. Sorry for all the tl;dr. I'm very bored and have nothing to do. =\