Davies will have two years left on his contract come the summer, and he hasn't yet improved upon the one he signed when he first arrived. Personally, I think he's off come the summer given those circumstances - it's the last chance for us to extract maximum value from his contract (besides January 2018, anyway) in any sale, and he'll want to move on to become a guaranteed starter somewhere.
The problem with him leaving is that, if KWP *does* replace Trippier, then it'll lead to a situation where both our back-up full-backs (because we'll buy a new one to replace Davies, no doubt) are inexperienced in the team's style of play. That gets worse if we also let Wimmer go in the summer.