Once Manu & Tim retire/ take a much reduced package in their "real" twilight years post 2015 (Duncan more likely than Manu), there would definitely be enough space to squeeze in Parker, Splitter, Leonard, Green alongwith a very good FA who earns near max money. I suppose that is one of the options the Spurs are considering for their post-Duncan project, even as the more likely option of blowing it all up and get the draft route is still available.
Even if your point of keeping a larger core with another decent FA is well considered, Josh Smith is certainly not the answer (and not just because of his erratic and pathetic play at SF/PF over the past two years, for which he doesn't deserve the contract he got]. That is because he will be earning near-max money in 2015 ($13.5 million). Instead, the Spurs could pitch the same salary numbers (cascading and increasing as years move on) to someone like Love, who is a much better fit. By mortgaging its future by tying up with Smith till 2017, the Spurs eliminate any chance of putting together a better core that you argue for. It is senseless, stupid and completely out of character with the most successful franchise in professional sports in the US since 1997.