Exactly. In the previous seasons when Enzo Maresca had not joined Chelsea, the team's performance was very bad and did not find the key that should have been obtained. After Enzo Maresca joined Chelsea, he focused more on getting new players who were able to improve the performance of the attack line and defense line. So as a coach, the team will definitely prioritize something more important. So maybe the coach thinks that if the defense is strong, the goalkeeper will never worry about conceding.
With this I support your opinion that blaming one party is unfair. Because every successful team definitely needs a process and the first process was successfully carried out by Maresca. Furthermore, the coach must also have a plan to find a better goalkeeper slowly.
That is right before Enzo Maresca joined Chelsea team did not play well and had trouble finding their rhythm. But after Maresca arrived he made it priority to make team attack and defense stronger by bringing in new players. As coach Maresca probably focused on most important things and maybe he thought that if defense was strong goalkeeper would not have to worry as much. I think you are right that it is not fair to blame just one person. Every successful team needs time to develop and Maresca has started that process successfully. Also it is likely that coach has plan to find better goalkeeper slowly which will help team play even better.