I think we're looking at the concept of "full agency" differently. In my view each person has their own level of agency, or willpower if you will, that is mostly biologically determined. I do believe that people can exercise restraint and grow their ability to self-regulate, but only to a point. And, it is quite difficult to do.

Quitting smoking is one of the most difficult things for someone to do. Some people can go cold turkey and power their way through it. However most people struggle because of their body's biochemistry. When discussing this topic I've noticed common responses along the lines of, 'that person didn't actually want to quit', or 'that person didn't want to quit strongly enough'. Yet I see it as their limited agency was unable to overcome their body's physical dependency.