That is a pretty fair point and one I always bring up about using advanced statistics and statistics in general so it's only fair I acknowledge that and I do agree currently there's 2 guys (to me the other is Durant might be different for you) who could turn any team into a contender/playoff team. But based on recent history I would go with the likes of Curry/Harden/PG13 (the latter two carried average rosters to playoffs in particular Harden's first year in Houston) but I just have never seen Cousins as the 2nd best player in the L and I personally think there's a gigantic gap between KD/LB23 and everyone else.
I agree that Durant is the other player. I am not sure that I would take any of those guys that you mentioned ahead of Cousins, not because I'm sure that Cousins is the third-best player in the league, but because I have never seen Cousins playing on a team that was built to complement what Cousins is good at. That Kemba Walker example wasn't totally random; I don't think that he and Curry are that far apart, talent-wise, and I'm certainly willing to believe that there's an alternate universe where Walker gets drafted by a team that subsequently drafts Klay Thompson and Draymond Green and Harrison Barnes, and trades for Andrew Bogut, and signs Andre Iguodala and Shawn Livingston and Marreese Speights as free agents, and is coached first by Mark Jackson, and then Steve Kerr, and he becomes a two-time MVP of the league.
Harden, at the risk of mixing metaphors, is basically the Ultimate Form of Tyreke Evans, and I feel like there's a cap on how good a team can be, if that guy is your best player... unless you build a team around him like Larry Brown and Pat Croce built the Sixers around Iverson: one other ball handler, two catch-and-shoot guys, and then nine other guys who basically don't do **** but rebound and play defense.
George? Eh... the year that Paul George broke out, while Danny Granger was injured, he still had David West, at the end of his prime, and George Hill at the start of his, as well as a motivated Roy Hibbert and a motivated Lance Stephenson... I don't know how "average" those teams ever were. If anything, you can argue that they underachieved.
I'll tell you who has taken an average team to the playoffs, though, and it's somebody who doesn't get a lot of love on KF.com: Chris Paul. In 2011, Paul took David West, the ghost of Peja Stojakovic, and ten guys named "Willie" to the playoffs, and pushed the gd lakers to six games. That's mildly impressive.