He will have left the She-gulls by Xmas when an average Premierships club come knocking
I don't think so. While he's got a chance to achieve something at Brighton he'll stick around. Of course if a Liverpool or a Man City sized club came knocking, then he'd have to think about it. But he's got plenty of managerial time for Premiership, European and International achievements.
Why not stick with a team you've helped start to build from the ground up..?