You fail to address why Detroit would want the overpaid, always injured Webber. I would think they'd actually want to improve their chances of winning another championship, not decrease their chances. Nope, I'm afraid Sacramento is stuck with Webber.
I think a case "could" be made.
1.) Rasheed is a FA, and to retain him they will probably need to sign him for 7 years. Webber only has 4 years left. So Detroit gets out from under a big contract 3 years earlier.
2.) To make the deal work Detroit would have to send 2 contracts (because Webber is making more than Sheed can get paid in his first year of a multi-year), so they can move a bad contract in the process.
3.) Detroit's defense improved when Sheed arrived, but with Prince, Big Ben, Billups, and Hamilton they still have a very solid defense, and replacing their PF with another good PF will not "kill" their defense. They may not be as good defensively, but they will be better on the offensive end.
4.) Webber is a great passer, and he would match up in a fabulous way with Billups in the pick and roll, and with Hamilton moving without the ball.
5.) Webber gives the Pistons a better offensive threat, and a better rebounder than Sheed.
6.) Webber is from Detroit, so coming home "may" rejuvinate him.