Objectively:
* His Per 36 scoring numbers are the lowest since 2012/13
* His per 36 rebounding numbers and REB% are the lowest of his career
* His FG% is the lowest since his rookie year
* His Free Throw Rate is the lowest of his career
He's a different offensive player now than he has been. His average shot was 12.3 feet away from the basket, because 24% of them were from 3-point range. Including this season, his career average has been 9.7 feet away from the basket. That is a HUGE difference.
Accordingly, his FG% dropped. However, his eFG%, which accounts for 3-point shooting, went up this year from last year, and was above his career average.
Because he is taking more shots further away from the basket, he gets fouled less, and so his free-throw rate drops. That's a function of his changing skillset, but not necessarily showing a decline. Being able to add an extremely valuable skill like 3-point shooting outweighs not drawing an extra foul at the rim, because it creates more spacing and thus better shots for his teammates.
His per-36 scoring was the highest third-highest of his career. His highest, in 2013-2014, was not in a full season, and therefore may be artificially inflated due to a smaller sample size. He also averaged his second-highest assists per 36, meaning some shots became passes for his teammates made baskets. Offensively, he's been about as valuable as ever.
While he may be getting older, he's already developing skills that will serve him well in aging, by being able to play further away from the basket, thus preserving some energy and health that can be expended by playing on the block all the time. Still, he shot 74.5% at the rim, and took 30% of his shots there, so he's perfectly capable of going down low.
Yes, his rebounding has decreased, and there may be a little worry there, but offensively, he's not lost any significant amount of effectiveness.