We have no shortage of resteraunts even when servers, dishwashers, and cooks are all paid $15 an hour.
Arguably most competent endeavors are capable of paying minimum wage.
Cratering the truly incompent is a service to the market and reallocates resources to more capable players and retaining a minimum wage fosters automation and forces capital to give a larger share to labor. Either is desirable. I understand the concerns you have about automation. They are potential not fate.
> I don't think a minimum wage is ethically justified in a minimum standard of living society
The owner class does not make decisions based on what is ethical they act to secure their own benefits everyone might consider doing the same.
> Once participation is voluntary forfeiting minimum wages seems like a very promising bargaining chip against capital to get a negative income tax / UBI actually put into law.
In a democracy we ought to be able to vote our own interests as soon as enough people see the merit of the idea. Fuck negotiations. If democracy remains in truth eventually inequality will make selling this idea a trivial sale beforehand its unlikely to come to pass here in the US.