Anderson isn't helping anybody anymore. He's no more a PF on defense than Pau is, and giving him minutes will make it harder for Gay to do his job. Spurs already have a more ideal Anderson in Bertans. You're also continuously under the impression that the Spurs have a lot of financial flexibility next season if they take on salary like Anderson. They don't. They'd have to make pretty tight decisions if they did this trade and paid the full MLE next year. It would depend on the Spurs' draft slot to see if they had enough to pay Gay even what he made this year -- and that's assuming the Toronto pick goes out. The only way to get breathing room would be to cut Anderson, but you still are in a situation where you'll have barely any room under the tax. To get real space, you'd have to stretch him, which goes against your idea of it only being a one-year thing.
http://www.shamsports.com/capulator?...268de797855470
To be clear, the Spurs could work around that salary. They could play hardball with Gay, or they could stash their pick, or they could not use the full MLE. But those are real compromises the team has to make. This isn't painless, and if the team is giving up a first too, it has to be worth more than just a middling SF.