I'm not sure how Artest's aggregate skills make him a viable choice to defend Bynum/Gasol or Shaq/Amare, if that's the biggest concern that faces the Spurs. In fact, acquiring Artest immediately gives the Spurs three guys who play the same spot and are more effective defenders than scorers (Artest, Bowen, Udoka) while at the same time relieving them of another big man (Elson, ineffective though he may be) who could at least offer up some token resistance in a matchup with a Gasol or Amare. And if you're going to keep Artest on a short leash, you HAVE TO keep Bowen and Udoka around, just in case the absolute worst happens. So, arguably, instead of dealing with the very thing that you've identified as the most significant concern, you've weakened yourself at that spot in favor of acquring a redundant part with a big name.
Certainly the Spurs caught a break or two in the 2007 Playoffs, but as you're so fond of saying, this isn't 2007. The Spurs might have needed a guy like Artest to deal with a smallball Mavs' lineup last spring, but they won't face the same sort of lineup this spring if the teams meet again. This spring, size is back in vogue in the West and getting Artest (instead of a guy like Thomas or some other big) essentially means that the Spurs will be forced to play smallball lineups with great regularity. I don't know that many Spurs fans would favor that idea -- particularly because it ensures that Duncan will spend long, long minutes defending Shaq or Bynum or Yao.
Again, I can certainly see an argument that the moves by Phoenix and LA might necessitate a reshuffling of the roster, but I don't see how those moves suggest a countermove to acquire another wing player. I think that if those moves are the impetus for doing something to the Spurs roster, the only logical move is one to get a big man.