Spurs won't do this deal on account that Splitter and Leonard are their solid future players. They may not be big names but they are the future, and no Kawhi would be more than a Kedrick Brown.
Pierce is an upgrade for RJ, but Wilcox and Pietrus is not for Splitter and Kawhi.
Spurs would not do this deal. I like the idea of having Leonard for us though. Flex 2/3 who can defend, shoot and slash.
Spurs have a core of old guys. Duncan has maybe one or two years left. Pierce is still a valuable player. The Spurs would unquestionably be a lot better with Pierce in the starting lineup over Jefferson, and while Pietrus and Wilcox are not as good as Leonard and Splitter, they're decent enough to make this trade a win overall for the Spurs from a win-now perspective.
I think this trade would give the Spurs the chance to really fight for the conference title against the Thunder, Mavs, and Clips. As it stands they're a nice regular season team, but I think they're a piece short for a real post-season run. To me, it would make sense for them to cash in some of the young assets they've accumulated over the last couple of seasons to upgrade their starting lineup.
The problem San Antonio has with this deal is, not a lot of FA's sign with this team. Wilcox and Pietrus are expiring deals, who will they have as replacements when their deals are done? Thats why they wont do it. Plus, the Spurs are contenders with this core, with Manu still being injured. They won't give up their developing assets just like that if they are contenders.
Spurs are contenders right now, but I think even they would recognize they're in the second tier compared to teams like the Heat, Thunder. If they can upgrade the top of their rotation for the playoffs in a big way (which I think they would in this trade), I believe they would strongly consider it.
Free agents have joined the Spurs when they've been at the top of the league. If the Spurs make a deep run in the playoffs with Parker / Ginobili / Pierce / Duncan this season, top veteran free agents would consider signing there. The Spurs still have a lot of reasonably young depth on the team even without Leonard and Splitter, too (Blair, Neal, Green, etc).
Splitter isn't really much of a prospect, he is already 27.
Splitter may be 27 already, but he's only been in the league for a year and a half and he's already improved immensely. Given his size and skill level, I think there's a good chance he could be a very solid player in this league for 8-10 years.