Definitely, if the Spurs throw in their '12 1st, that would more than give the Kings incentive. I wouldn't do it for Thompson, though. I realize the Spurs need a tall big, who can shoot and preferably a young one, who's not overly expensive. Which narrows their options considerably. But Blair had a 17.20 PER last season. Better than Rondo, Marion, M. Gasol and Bogut (though he did play injured), to name a few. Thompson had a 13.59 PER (league average is 15). Sure, it doesn't account for defense, but Thompson is a sub par defender too and he's 3 years older. In other words, Blair is the better player now and has a higher upside. Throw in the 1st and it becomes even more lopsided.
Thompson would be a better compliment to Duncan offensively, but he wouldn't help defensively, which is their supposed goal. It doesn't have to be a big with freakish athleticism, just someone who's at least slightly above league average athletically.
It's not even improbable, much less close to impossible. I'm confident the Spurs have the assets to acquire Thomas if they want to. Is he a "high quality big"? Not quite. But he had an 18.25 PER last season, so he's trending in that direction. And at his age (25 later this year) it's possible there's still upside left, despite him having played five seasons already. He's also the type of athlete/defensive presence that has been lacking next to Duncan.