The only argument is that having to beat LeBron and then ONE of the Western Conference contenders is an easier road to a title than having to beat 2 or 3 Western Conference contenders and THEN beat LeBron on top of that.
No team in the East can offer Durant the level of talent his current team already has. But playing in a conference with only one other team that has any hope of beating you in a series means your odds of winning a title are much higher.
But talent without fit also doesn't mean much, as OKC itself and the Cavs have proved this postseason. One of the best arguments for him leaving is that he's not a good fit with Westbrrook, so I think there are several more reasons than just being in the East as you argue.
Sure, they might out-talent many teams on most nights, but a poor fit together is exposed when you play truly elite teams. Golden State is highlighting the poor fit in Cleveland right now, despite the immense talent that they have.
The Thunder beat a 67 win Spurs team and came within one game of closing out one of the greatest teams of all time. Closer, by far, than anybody else this season, barring an astounding turnaround in this Finals series.
That's good enough for me. If I were advising Durant, I'd tell him to stick around in OKC unless he believes Russ is likely to leave in a year. Even then, sticking with OKC might still be the best option. I don't think a team that signs Durant this summer is likely to offer him a significantly better chance at winning a title in the next 5 years.
Eh, I take the Spurs series win with a grain of salt. Not only did they vastly underperform that series, OKC is also a bad matchup for them. Further, though it's nice that they challenged a Warriors team with a Curry that was recovering from injury, they ultimately fell once again due to the knock that has always been present with them - the inability to finish things and the propensity to choke, which is LARGELY due to a mismatched roster and poor fitting pieces.
I also think there are several doable scenarios where we can put a better team around him that is more likely to win a title than OKC. I think you're underestimating fit and chemistry a bit. They've had the talent to win for a long time, but they haven't won anything because their ceiling is just limited due to that bad fit. Cleveland is a magnified example of OKC - all the talent in the world, but they're trying to make mismatched pieces fit together.
Look at the teams that have won the title recently - Warriors, Spurs, Heat, Celtics, etc. All of those teams' main pieces fit together seemlessly. Look at the teams that came just short due to poor fit - OKC, Cleveland, Houston.