I think questioning his decline is fair, as he has been slowing down over the past 2-3 seasons now. A lot of his play in general seems to be almost situational picked rather than playing with a high motor at all times. But I think that just comes with age. I thought during the playoffs he has still stepped everything up a notch where nobody but maybe KD can really reach anything close to that level, but he's not in the playoffs this year to really determined if that has declined or not.. I think like you said though, the decline is more inconsistency effort wise, and the young personnel around him, the injuries, line-up changes, and the team trying to get that chemistry on the fly only made that worse.
http://www.backpicks.com/2017/12/11/...n-nba-history/
I don't agree with everything on his list, but the individual breakdowns for each player are definitely worth the read when you get a chance.
This is also a list is also based on their careers, so it's not really just ranking guys at their peak which makes a few questionable names pop up but they make sense since the criteria is "40 best careers" not necessarily "40 best players".
Like I said, even ignoring the list itself altogether, just reading each players scouting report makes you really appreciate the amount of time that was put into this. A lot of it is heavy in advanced stats and analytics, but unlike most he has scouted videos that go with them and IMO does a solid job of bringing the whole "eye-test" argument and stat argument into one whole piece.