Lets count the reasons:
The value you get back from him in return may not even out. Isn't the talent you have better than a prospect? Is the franchise giving in the towel for the next four to six years? While it does eliminate salary you're also decreasing the value of the team. A team that is up for sale.
I thought when you wanted to sell you wanted to increase value? Having Adrian Gonzalez and no other productive talent on your team doesn't make me want to put together an ownership group and make an offer.
Peavy needs to approve the trade. If he doesn't want to go to that team he can block it. This is a brilliant and very worthwhile clause for a player. If only Curt Flood had it in 1969. Flood refused to accept a trade sending his case to the U.S. Supreme Court. He ultimately lost but brought about change in baseball's free agency rules.
The loss of San Diego's ace will cause fans to look for other spring amusements in sunny Southern California. I'm a transplant like many residents of San Diego. My job and life kept me busy enough last year to have me miss going to a single game at Petco Park in 2008. Maybe I'll start following teams from Los Angeles and Phoenix.
With MLB Network and the MLB Extra Innings package I can go all season not seeing one Padres game.
With these negatives the only possible upside would be that you get one or two outstanding players for a proven perennial All Star. Is it worth taking the chance?