What are the precedents for a top-5 in-his-prime player in the league being traded? Some candidates (numbers are previous year's WS) might be:
30yo Garnett (10.7) --- for --- Gomes (4.2), Green (1.6), Jefferson (6.6), Ratliff (0.1), Telfair (0.5), and two 2009 1sts.
31yo Shaq (9.9) --- for --- Caron Butler (2.1), Lamar Odom (8.4), Brian Grant (4.9), a 1st and a 2nd.
31yo Wilt Chamberlain (20.4) --- for --- Archie Clark (7.9), Darrall Imhoff (4.9), and Jerry Chambers (1.4).
27yo Kareem (12.9*) and Walt Wesley (0.2) --- for --- Junior Bridgeman (just drafted #8), Dave Meyers (just drafted #2), Elmore Smith (4.7) and Brian Winters (1.1).
* this really doesn't represent his value since he missed a bunch of games that year. The 4 years prior, he'd averaged 22 WS.
I'm really not sure that there's all that much precedent for trading a 26yo Kawhi, but I guess the Kareem trade would be the closest.