I stated from the start that lower density smaller cities (which Copenhagen or Amsterdam are - having density 1/3rd and 1/4th of San Francisco for example) may allow for luxury of letting people to enjoy their commute on bikes.
High density usually isn't the problem for American cities (though it might be for SF, I don't know that one specifically), but rather low density suburban sprawl is. Higher density usually makes it easier to walk or use personal transport like bikes.
For it to be too dense for biking to be a viable transport they would have to be much denser than European cities which would by definition not allow for parking in the places where business takes place (in which case nobody would drive).
Most American cities were clearly designed with cars in mind, while most European where not (having grown organically rather than being designed), and while the American decision might have seemed better for the better part of the last 100 years, that might not be the case for the next couple of decades.