Did I say mandatory? I meant optional! You’re “free” to die in a cardboard box under a freeway as a market capitalist scarecrow warning to the other ants so they keep showing up to make us more!
Precisely. The medium of value delivery is irrelevant, as soon as you extract value by borrowing against an asset you have completed a transaction and therefore is a point at which it could (/should though that’s the debate I guess) be taxed.
In both cases (payment in bitcoin or borrowing against stock) your remaining position could go to zero leaving you liable for tax you don’t have money to pay, but that’s on you to manage better.