He will be making 9.5M because he scored a ton of goals last year, but even without the high scoring rate this year, he's still on 44 point pace without any powerplay time. He's probably a 60 point defenseman if he's on the powerplay. Plus he brings it on the PK, hits like a tank, skates like the wind, stands up for teammates, etc. Like I said, check out who actually broke the defenseman market with the signings that happened before Nurse was signed. But Nurse will go forward as "overpaid" because he was paid in part due to his goal scoring last season, which will drop this year, while nobody recognizes that he is still going to put up 40 -45 even strength points this year.
"Average" top pairing isn't second pairing either even if it was true. The guy is a #1 defenseman.