Arenado was dealing with inflamation in his left shoulder AC joint last season, which was apparently the culprit for his diminished power output. From what I could find it just required rest, no surgery. From Arenado's first All Star season in 2015 through last year, he totaled 27.9 WAR, 8th most per Fangraphs. How did he get there? It all starts with playing time, Nolan had the 7th most PAs from 2015-20. He'll likely play considerably less often in his thirties. On a rate basis, his 125 wRC+ ranked 44th, but since played so much, he ended up with the 23rd most batting runs. The main thing driving his offensive profile is the power tool, with his ISO+ of 161 ranking 7th over that time frame. If Arenado's power slips from elite down to even just very good, his offensive profile will take a pretty big hit. Defense typically declines with age too, but true elite defenders like Arenado (+90 DRS / +38.7 UZR since 2015) seem to be able to hang on a little longer, so that aspect of his profile would give me the least concern. Ultimately, two of the three things from which Nolan has derived most of his value (health & power) have some legitimate question marks moving forward. The Cardinals were apparently the only team willing to pay 165 million to live with the answers.