it is opt in until they manage to convince some government to allow them to be the contracted provider of "humanness verification" that is then made a prerequisite to access services.
Comcast is also opt-in. Except, in many areas there are no real alternatives.
I doubt Worldcoin will actually manage to corner the market. But the point is, if it did, bad things would happen. Though, that’s probably true of most products.
altman building a centralised authority of who will be classed as "human" is about as authoritarian as you could get