Kevin Garnett - His minutes per game dropped last season after being pretty consistent through his career... And his average rpg and apg have been slowly dropping the last 3 seasons
Ray Allen - His PPG dropped to 17 last season after being pretty consistent over his career at 23-25. You have an argument here.
Jason Kidd - His PPG have dropped steadily since his career high of 18.7 in 2002-2003. Still, another good argument.
I don't really know basketball that well, so you may be right.
Keving Garnett's performance this season has been consistent with his best seasons, on a per 48 minutes basis. The same is valid for Ray Allen and Paul Pierce. The drop in their production is not related to skill decline.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."