I think your question can certainly be answered in more than just one way.
"Do we keep Skal?"
Right now, as in, do we keep him past the trade deadline coming up in a few days? Absolutely, we do. Because we're probably not going to get much in terms of value in return.
Do we keep him going into the future? That's a black and white issue to me, and it is not only a B&W with Skal, it is the same with almost every "tradeable" player out there, on any team. You always look to move your pieces if it's the right move. I don't think everyone, in that sense, is virtually untradeable. I honestly believe that you can move every player on a given roster for the right pieces in return. Even a guy like LeBron can be traded if Cleveland gets a crazy package in return. I am sure the Spurs would have also traded Tim Duncan in a heartbeat if the other team offered them 10 future 1st round draft picks, a few thousand dollars in cash, and about 50% of their roster.
Now...Am I saying that the Spurs would have traded Timmy? No, I am not. Timmy was the Spurs. The Spurs were Timmy. I don't think he would have ever agreed to wear another team's colors. Nor do I think that Pop would have allowed Timmy to wear another team's colors. All I am saying is that, for the right offer, even a guy like Tim Duncan is not considered "untouchable".