I mean he's pretty unquestionably improved on last season in terms of shot-stopping, that's layed out in the data. Last season he was 5th worst in terms of PSxG/90, this season he's 4th best. And last season itself was pretty obviously an improvement on the previous couple of seasons in terms of glaring individual mistakes. You'd have a hard time making any reasonable argument that his form hasn't improved, particularly given he was a walking nightmare at some points over the last few seasons.
The issue is that all this improvement occurs within the framework of the type of goalkeeper he is, namely a goal-line keeper who relies heavily on his shot-stopping because he's not good in possession or at chance prevention. He could be in form and acting as a very good version of that keeper, but being that keeper is still a problem. And while he can make minute changes in terms of getting better at sweeping or whatever, he isn't going to suddenly become a different goalkeeper. That's why he needs to be replaced, not because he hasn't improved.