Prefacing this post with the fact that I'm actually a Dubs fan, and I do believe that Curry is in fact the best player in the league. But there are a couple of things that need to be debunked.
Steph Curry is absolutely the best player in the NBA right now. The difference between Steph Curry's PER and Lebron's PER is the same as between Lebron and Kenneth Faried.
PER is a very limited stat in that it
only measures box-score contributions, and ignores stuff like picks, off-the-ball movement, positional gravity, close-outs, box-outs, etc. It also ignores who your teammates are, which is kinda important.
I know the quants at Toronto & Boston, and they laugh at the notion that PER is a useful metric.
Lebron is arguably the greatest player of all time and definitely top 5, but he his athleticism has slipped and he is not the same player he was 5 years ago, particularly on the defensive end. The advanced metrics actually favor Curry as a defender over Lebron.
This is really, really untrue. LeBron's DRPM (defensive real +/-) is +2.62, which is the second-best of any wing player in the league behind Kawhi, and 36th overall. Curry's is +1.29, which is 91st in the league and fourth among point guards.
DBPM is even more lopsided (defnsive box +/-), with LeBron at +2.3 and Curry at +0.1, placing LeBron in the league's top 20 (third best wing behind Kawhi and Danny Green) and Curry slightly above average. Defensive win shares is closer with LeBron at +2.4 and Curry at +2.3, but win shares is a pretty crappy stat.
I don't think any of these stats are perfect, and generally speaking Curry's been really good defensively this year while offensively he's reinventing the game.
That said, people have gotten so used to LeBron's greatness on both ends that we don't even appreciate it anymore -- and that's sad considering he's, at worst, the 3rd-best player of all time.