3. Goaltending is a bad investment
If a goaltender's success is more predicated on the team's defensive structure than anything else, then why invest so significantly in the position? The Islanders have $11M AAV invested in Sorokin and Varlamov and are getting a mediocre return on that investment.
Goaltending is a difficult position to predict future performance in, and the difference between great goaltenders and average goaltenders is negligible. Given the position's volatility, maybe high-AAV, short-term contracts should be good business, but they are rare. Top goaltenders earn the same long-term contracts as skaters without as much certainty in sustainable performance.
Sorokin fits into that bucket.

The other problem not specific to Sorokin is that goaltenders tend to get worse with age, with sharp dips in play starting after age 33. The Islanders have Sorokin signed until he's 36 and unlike a big salary forward that you can move down in the lineup, he'll have to be in the starter's net or on the bench with a big hefty salary that can't be moved.
The best-case scenario for Sorokin and the Islanders is for the team in front of him to improve. However, with 13% of the salary cap going to goaltending, it's made more challenging. The Islanders are looking at attempting a re-tool in the coming years, and Sorokin is among the core pieces that will be counted on to make a quick turnaround possible. However, the past two years have proven that if you want your goaltending to be great, you need a great team more than a great goaltender.