Let's try it from Sac's point of view.
-You have a rookie that you drafted #5.
-You are still rebuilding.
-You're rookie is 20 years old. Tons of room for development.
-He's 6-11, 270.
-As a raw rookie, he's already stepping right in and giving you 11.7/7.2 in 24 minutes.
So now Boston calls you and wants to trade you:
-A guy on a one year contract that everyone in the league had a chance to grab, a guy who had to spend time overseas because no one wanted him.
-A rookie everyone passed on.
-A role player who is about to be an unrestricted free agent who is still averaging the same points and worse rebounds in 4 more minutes a game and who is 4 years, 8 months older.
If you were Sacramento, would you even consider this deal for a single second? Absolutely not.
If you were boston, and the Sac guys got ridiculously drunk and somehow offered you this deal, you would get their signatures before you could sober up. I don't care about the "intangible" immediate dropoff, you'd instantly be getting better scoring (look at the touches Davis and Erden get around the rim on this team and imagine Cousins working with those touches) and much better rebounding. And you'd have a probable 20-10 guy in a couple years, a chance at a true franchise big.