Gotta say I find Kos' quotes troubling, and I was surprised by that. Much as I hate to admit it, there's more than a tad of RVCuntishness in those quotes, and I think we could expect him to leave at some point.
Having said that I think there's zero chance of him leaving this summer. For all the "we just need the right offer" talk, every single player we sold in the past had some reason for selling them (doesn't mean you have to agree with them or anything). There's none for Kos - try as you may, you won't find one. It is almost as certain for next year too, but what goes after that is anybody's guess.