If you're a remote working who has been living and contributing from NYC for 5 years, what is the justification for a company giving you a pay cut because you move to Kansas? Your contributions remain the same. The responsibilities remain the same. The costs incurred by the company remain the same. You may even keep the same hours by adjusting your working hours in the new time zone.
I don't see a justifiable reason for it either direction. Living in an expensive city is a choice, now more than ever with more and more tech-focused companies moving their devs fully remote.
It's not about costs, it's about supply and demand. If you move to a lower cost/tax region the amount of money you're willing to take to do the same job is lower.
You can say "no" to a paycut. And they can say "no" to keeping you employed at the same rate. Eventually you'll settle on an equilibrium - the intersection of the supply/demand curves.
It’s not about supply and demand - that makes no sense. That only makes sense if the company’s alternative is to hire a new employee at the Kansas City wage rate - but that option was never on the table. The choice is between retaining an existing employee that you’ve been happy to pay a current NYC wage.
If you could have hired a Kansas City employee to do the same job for less pay, that would have already been reflected in the previous salary negotiations when you instead decided it was worth it to pay more to hire the NYC person.
That person choosing to live in a different location has no effect on anything - everything that led you to the original choice to pay them a higher wage remains true.
I don't see a justifiable reason for it either direction. Living in an expensive city is a choice, now more than ever with more and more tech-focused companies moving their devs fully remote.