Defensive rating is completely misconstrued because the league is offensively so much more skilled now. The last 6 NBA champions all are among the worst 10 defensive ratings of NBA champions because the game has changed. I’ll need to see other analytics to prove your point.
The league is down is a croc in my opinion too. The league is more skilled now than it’s ever been.
The best player in the league has the best career PER of all time. 5 of the 10 best career PERs are currently playing (Jokic, LeBron, Durant, Anthony Davis, and Giannis). With the growth of the game internationally you have a larger pool of better players entering the NBA. In my opinion, you are mistaking parity (which is good for the game) as the league being down.