Aasgier, I never found any of the Pokemon you mentioned impressive in the first place, besides Gardevoir.
Arbok, while it could set up easily thanks to Intimidate and Coil boosting defense, was dependent on setting up and needed two boosts before it could really get going. +1 Arbok really didn't hit very hard as long as you resisted Gunk Shot, but even if you didn't, you could usually take a hit if you're bulky and at good HP. Arbok's base Attack was only 85 and it had to use Black Sludge instead of a boosting item, which made it pretty weak until it obtained multiple boosts. Arbok also relied on Sucker Punch to be faster Pokemon. In addition, set up sweepers not named Swords Dance Samurott, Swords Dance Scolipede, or Rain Dance Ludicolo tend to be poor in NU.
Pinsir was always a Pokemon that performed much better in theory than practice. Its strongest STAB being X-scissor made it surprisingly weak in many situations, it was slow, and very vulnerable in many situations because of its low Special Defense stat and numerous weaknesses. Pinsir also had some 4mss issues, as it wants X-scissor, Earthquake, Swords Dance, Quick Attack, and Close Combat. I also never really found Choice Scarf Pinsir good after the initial hype... it always ended up being walled by a lot things depending on what move it locked into so it couldn't effectively clean, and there are better revengers in the tier like Choice Scarf Rotom-S. Golurk being one of the best Pokemon in the tier was also not very nice for Pinsir, as it can easily tank any attack. That said, Pinsir is still pretty decent now.
Butterfree was (is) a pretty cool Pokemon. I don't think its viability was changed that much, although all of the Choice Scarf users we see today easily destroy it. It basically only has one chance to sweep with Quiver Dance, and it's easy enough to stop a sweep from happening. A support set is also pretty nice, but I question just how much utility it has. It should be decent enough to make a viable team with, and it still has the flaws it has always had.
Leavanny was kind of mediocre anyways. It has its uses but they're not so amazing.
Volbeat (you didn't mention it but it was talked about in this thread and it is a Bug-type) is really limited in what it can do, but it does it well. It's a good sun supporter and Tail Glow pass is cute, but never really works out as well as you'd want it to.
I don't think this metagame has hampered creativity/diversity much at all. Sure some Pokemon fall down in usage because they are somewhat out shined, but that's how a metagame works. Some Pokemon will increase in usage because they handle the top threats better, some will decrease. A little centralization isn't bad for NU either, but I don't think the new threats have really centralized the metagame in any way, except maybe Scolipede. You tend to naturally have a lot of checks to Jynx while preparing for other threats, and if your team happens to be really weak to Jynx in teambuilding, you can just throw on a Metang and it should work fine while checking a lot of other annoying stuff like Exeggutor.