I agree that Nifty probably has more potential than Tarik (Nifty is only 20 years old), but right now I think Tarik is a much better player.
Nifty is pretty good, but he's not a superstar AWPer. The main reason I think he's so good is that he's an above-average pro AWPer AND and above-average IGL. But with nitr0 and Tarik, I'm not sure Nifty is a big improvement as an IGL.
As an AWPer, while nitr0 isn't great, he's at least an average pro. Stewie is streaky, but probably also almost as good as the average pro. Tarik is probably a little bit better than the average pro. Imo, Nifty is probably only a bit better than Tarik as an AWPer, while a hot Stewie would be better than them both.
The other benefit of adding Tarik is that it gives the team a ton of flexibility.
If the strats are bad, they can change IGLs. While Tarik and nitr0 are the best IGLs, Stewie and autimatic have also been IGLs at some point in their career. More importantly, all of them extensive experience as non-IGLs. Nifty has never played at the top level as an IGL, so it would be harder to let someone else lead.
Also, if Stewie, nitr0, or Tarik were struggling with the AWP, they're all good enough with rifles that it wouldn't be a big issue to have them switch to a rifling role, keeping them on the team, which gives them a chance to improve their form. If Nifty was struggling, he isn't good enough with a rifle to fit into a superteam, so you'd have to cut him.