Semantics
PUBLIC ENEMY #1
The problem is the tax rates can change at any time, whereas NHL contracts last up to 8 years. You could set a different conversion rate per team every year, but that sounds like an accounting and legal nightmare for the teams. The tax laws are also so complicated and full of loopholes that I'm not even sure how one could create a sound legal framework for that type of contract.
People also can have wildly different personal tax situations. A player making minimum salary is going to have a smaller tax rate than a $10M player. Some players might have huge deductions, then what? They may be incentivized to sign in places with high tax rates if they can take advantage of deductions to net more than they would elsewhere. There are ramifications for things like mortgages, donations to charity, or side businesses/investments.
I think you'd just end up where players prefer still certain markets, just different markets and for different reasons, and having added a shitload of complexity and legal exposure to how the league is run.
tl;dr - Sounds good, doesn't work.
People also can have wildly different personal tax situations. A player making minimum salary is going to have a smaller tax rate than a $10M player. Some players might have huge deductions, then what? They may be incentivized to sign in places with high tax rates if they can take advantage of deductions to net more than they would elsewhere. There are ramifications for things like mortgages, donations to charity, or side businesses/investments.
I think you'd just end up where players prefer still certain markets, just different markets and for different reasons, and having added a shitload of complexity and legal exposure to how the league is run.
tl;dr - Sounds good, doesn't work.