The reason why I concentrate on GA/60 is because you win games by goals, not expected goals. One thing I have noticed on Ceci through the last 5 years is that he maintains a very low GA/60 especially for his deployment while the advanced stats are not that pretty. At some point you have to say that whatever he does, its effective. 5 years of data just says its not luck anymore, its a trend.
Through the first three rounds Ceci had a 1.1 GA/60 without Nurse, Nurse was over 4 GA/60. It was obviously, in a huge way, that Nurse was dragging Ceci down, not the other way. Nurse is not cut out to be a defensive stopper, its just that simple. He needs to be moved to a role that suits his game. Ceci is a decent defensive stopper, and if paired with a good shutdown LHD, its a big bonus for this team.
Ultimately the team finished 9th in GA this year, 5th if you could ignore that disastrous 11 game start. For more than a decade the knock on the team has been D/G. At this point I'd prefer not to throw the dice on a massive shakeup on the defensive end. Its not like Ceci is making 7M, he is below league average. He played the third most minutes on the defensive end for a team that just went to game 7 of the finals largely by what they did on D.
Your post had me thinking so I looked at the GA/60 stats of all Oiler defensemen who played at least 1000 5 vs 5 minutes over the last 3 years. There were 8 of those. Ceci had the worst GA/60 of the 8 tied with Bouchard at 2.61. The problem though is that Bouchard had an on ice GF/60 of 3.33 to Ceci's 2.54. In fact of the 8 defenseman Ceci was the only one with a GA/60 greater than his GF/60. And in your own words you win games by scoring more goals than you give up.
Specifically with or without Nurse:
Ceci and Nurse: GA/60 2.56 GF% 51.33
Nurse without Ceci: GA/60 2.62 GF% 49.67
Ceci without Nurse: GA/60 2.71 GF% 45.38
With neither: GA/60 2.37 GF% 56.86
If you want to look at playoffs Ceci's GA/60 numbers do indeed improve without Nurse, but his GF/60 also craters since in those situations he is usually being banished to playing with and against low level players.
Ceci and Nurse: GA/60 4.19 GF% 43.75
Nurse without Ceci: GA/60 3.47 GF% 48.65
Ceci without Nurse: GA/60 1.86 GF% 47.37
With neither: GA/60 2.40 GF% 54.84
As usual these numbers are really just a snapshot and do require additional context.
I actually agree with you that it is a risk to "shake-up" the defense. And I also do not believe that Ceci at $3.25M for a #4 RHD is over paid. The problem is that if you look at the team and ask where would the biggest impact be from an upgrade it is hard to look past Ceci because Nurse is going nowhere. Unfortunately, the options for how to achieve that upgrade are slim right now. If the Oilers had the cap sapce to play Ceci and the third pair RHD I think he could be very good in that role. But the don't and as of now he is certainly a guy that may not be long for the team.