Hmmm, could I "retaliate" by getting a PO Box or mail-forwarding service based in SF or NYC while actually living in a cabin in WV?

They are benefiting from this just as much as you are. Remind your employer that he's also saving money by not having to pay for an expensive office space in the city center and all the things that go with it like hiring cleaning services or building staff. Any equipment you use to work on should be either reimbursed by him or factored into your salary as well (monitor, laptop, internet connection, office chair, desk, etc). There are a lot of bargaining chips, just depends on how you present them.

The more employers willing to hire remote workers, the less this argument works.

Still, I already make 1/2 as much as I could make living in Boston, living 90 miles north of it. That is "local market" salary though for a job that is ostensibly on-site (partial remote / now fully remote due to COVID).