did ever a championship team come out of the free agency? I can't remember. as Bruno mentioned, there are 3 way to improve: trades, draft and free agency.
but if we look at the championship teams of the last years, all of them were built by draft and trades. (I'm talking about the cornerstones, not the role players)
Spurs only by draft (Tim, Tony, Manu), Celtics, Heat, Pistons, Lakers, Bulls by a combination of picks and trades.
the last FA signing I remember that turned into a top three player on a championship team was Billups, but he wasn't a big money signing, back then he was the typical MLE signing.
all the other max signings out of cap room didn't turn teams into champs. very often they didn't even improve the team.
I think free agency is good for finding complementary players, but not for building the foundation of a big team.
the reason is simple. to be able to make a max signing, the team needs to be below the cap that far, that there usually isn't enough depth left. you may add a star, but to finish the whole building, the team usually doesn't have the potential left.
trades are a different thing. via a combination of contracts, you can get those players, that are crucial to finally win it all. see Shaq to Heat. KG+RA to Celtics. Sheed to Pistons. maybe Gasol to Lakers. maybe Mo Willams to Cavs.
that's why I'm not a fan of all the free agency scenarios.
if the Spurs in fact reduce the team to just Tony and Tim and a few rookie contract players, they will very likely find themself without a top FA and even if they do find a decent player, they would be to thin to compete. (we need to see that a big signing 2010 automatically means that Manu is no longer with the team)
improvement via trade is the much better strategy. and this will be the year to do it, there will be more star players on the market for dump packages than ever. Spurs might not be as lucky as the Pistons in 2004 or the Lakers in 2008, but a nice player can be all theirs, if they pull the trigger. I'm absolutely sure that another le can only be won, if the Spurs keep the big three together (pray for Manu's health) and add one significant piece. forget the 2010 free agency. 2012 will be the year to re start from scratch. till then they have 3 more shots. if they take some risk this summer.