It's true that Genesect's defenses are actually better than Scizor's (that's what you get for having the same BST as your previous evolution >_>) but find me a Genesect that actually invests in bulk. Protip: you won't. Nearly 60% of DW Genesect are good old 252 spa/252 spe. Ignoring the 20% other (because it can't really contribute to any percentage), there's a measly 2.1% that are noted for an HP investment, and not one bracket has investments in defenses. In comparison, the majority of Scizor invest significantly in their HP, and plenty of them in special defense as well. Out of the six EV spreads with enough usage to be listed in the june 2012 stats, five had HP investment, totalling over 50% of all Scizor - and that's not counting any rogues in the 30% others. 5% of all Scizor also sported notable spdef investment - hardly a groundbreaking amount, but still way more than genesect.
As for Tyranitar, I specifically said that Scizor's pursuit will outpower a BULKY tar. Offensive Tar has attack investment that, combined with STAB, will obviously beat Scizor's pursuit any day. But if the opponent flees, a standard (heavily invested) Scizor's unstabbed 80 power Pursuit smoothly beats an uninvested Tyranitar's 120 power Pursuit (252+ CB Scizor 80 power Pursuit is stronger than 0 Tyranitar's 80 power Pursuit boosted by STAB, by about 30%). If the opponent stays in, Scizor's got choice band AND technician on its side, so it'll still be stronger than uninvested Tyranitar (252+ CB Scizor 40 power Pursuit, boosted by Technician, is stronger than 0 Atk Tyranitar's 40 power Pursuit, boosted by STAB, by nearly 100%). But this is all academic. My point was that, if you compare the standard CB Scizor to 0 Atk Tyranitar (and those are hardly a rarity. according to
our most recent stats, they compose somewhere between 25% and 30% of all Tyranitar), Scizor's Pursuit is superior. In any case, it's one of two OU Pursuit trappers, so it's not like there's a lot of competition.
As for Scizor's bullet punch - you're right on that point as well. Many of the most recent threats, such as thundy-t, are resistant to bullet punch. This is part of mamoswine's popularity - in comparison to the horrendous offensive typing that is steel, ice is a fabulous attack. Scizor might have a power advantage over mamoswine (thanks to technician, CB Scizor's Bullet Punch has more power than CB Mamoswine's Ice Shard), but that's easily compensated for by ice's far superior typing. Since the beginning of CB scizor, it has only had one way to revenge things - its priority bullet punch - and anything that resists it will probably laugh it off. This has been an issue with scizor ever since it learned bullet punch back in the early days of dpp.
But while Scizor's job keeps getting harder and harder - kind of true for any older-generation mons, isn't it? - it still has an advantage over Genesect, Mamoswine, Tyranitar et al. Namely, it does ALL THESE JOBS AT ONCE. If you look at any one of CB Scizor's options, it's easily outclassed. as a u-turn scout, I'd rank mienshao's u-turn far above Scizor's because of its infuriating regenerator keeping it alive, and its ability to abuse LO for move switching. As a priority revenger, Mamoswine's typing probably gives it the advantage right now, and let's not forget that like a bajillion mons can carry choice scarf to instantly transform into viable revenge killers. As a bulky steel/bug, Scizor's never really been durable enough to play the walling game - at best it's a pivot, compared to defensive tanking attackers like slowbro or the occasional volt switch forretress (okay forretress as an attacker is a stretch >_>). As a sweeper, mons with broader coverage, better speed and less need for choice band are plentiful and they can compete with Scizor's bullet punch. And as a pursuit trapper, offensive Tyranitar has always had the edge.
But what mon does all these things for the low low price of one team slot? I think you'll be hard pressed to find one. That, imo, is what has kept Scizor at the top. It's the ability to fill so many team roles and check so many threats in one elegant red package. Introducing Genesect into the picture doesn't change the fact that, at any one of these roles, you'll always find mons that do the job better. Genesect is powerful and flexible and it's a significant mon among those. If you were running Scizor primarily for a role like u-turn scouting, you will probably find Genesect is capable of filling those shoes nicely, if not better than Scizor. But Scizor does more than that for a team - a lot more - and if those other roles (whether they be bp sweeping, revenger priority, pivoting bulk, pursuit trapping, etc) were important to you, I doubt genesect will be "outclassing" your Scizor any time soon.
tl;dr do I really need to write an essay on the most used mon in OU, just to explain why it's not outclassed by genesect (or any other mon)