The career span of an NHL player is relatively short, and MacKinnon is paying taxes at the 37% federal bracket plus another 4.55% for Colorado state taxes, so he's only saving towards retirement the remaining 60% or so, less current living expenses. Then, there are other expenses such as whatever percentage his agent is taking.
Also, it's not like MacKinnon will retire at 65 and only need the money for maybe 30 years, but rather if he retired at 35 he would need the money for maybe 60 years (what else is he qualified to do back in the real world? Be the captain of a 737? Take a job as a derivatives trader? Go into plastic surgery?). So, from his perspective, he needs to make enough money that he can live off the investment return on that money and also beat inflation over 60 years. How much is that? Well, it's obviously a guess, but the most practical answer to him is "As much as I can possible make."
Now, if you start looking at some of the players in other sport leagues who have quarter-billion player contracts, plus multi-million endorsements contracts, then "How much is enough?" becomes a moot point. But that's not the NHL.