Fair or not, if we get to another final, Karius will be playing his past. The build-up will focus on it (amongst other things), and it's not hard to imagine the opposing players thinking: 'I'm going to try him out with some shots, and shake him up, knock him about, unsettle him early on'. That baggage will be there in a final no matter how well he plays in the previous rounds. That just puts needless pressure on the whole team, and especially the defence. So it's fair enough to tell him that he starts anew this season, to push him to improve, but he still needs to be replaced. It's about the progress of the team, not him.
Whether we get a replacement now or later, as I said, I've no idea and no really strong suggestions once you leave out those who clearly aren't moving anywhere.
But money needs spending elsewhere this summer, too, and the club needs, at least, to show it really means business with the massive funds it now has to buy tried and tested talent.