Santana loses a ton of value in LF vs. SS or CF. Let's just say for a second that he's capable of posting a .850-.900 OPS. That's immensely valuable at SS or CF. In LF, there's a lot more people who can do that. If there's no room for him at SS or CF (and I highly doubt it if he continues to play the way he's playing as he'd be an all star), he's far more valuable in a trade than on the Twins.
That said, he's hitting well now, but I doubt that he's going to maintain an OPS around .850 as he is currently doing. His career minor league OPS is .702, and his best season OPS was in 2012 at .739 (though as a bonus in the pitcher friendly FSL). Given his age, outperforming his minor league numbers is not an unreasonable expectation and his increase power this year is also something that is sustainable given his age, but given his plate discipline and other tendencies, I suspect he's going to regress a bit, but posting a line between .750-.800 OPS over the next few seasons is far more reasonable. In LF, that's really nothing special. At SS, it's very valuable.
My take on Santana is that given his play, he's earned the 2015 starting SS role. I expect some growing pains defensively, but I think it's far better for Santana and for the Twins to let him have the spot. Escobar can move into a utility role spelling 3 IF spots. There's not much at SS in the high minors, so I think the position is his. The Twins are better off signing a decent LF or perhaps trading someone like Darnell or Gilmartin for one.