The employer deciding what "the market" is to set "market rate" to a conveniently lower number isn't reasonable. If you're hiring remote-first, you're not competing in a local market.
The employer deciding what they're willing to pay employees is eminently reasonable. Most of these companies are NOT going remote first, they're just _allowing_ remote work.
"If you relocate from our San Jose office to our Austin office, there will be a compensation adjustment due to COL and prevailing market rate."
I don't think that's unreasonable.
I'm sure they make exceptions for those employees who they want to retain at all cost.