True as that may be, I think the argument that resonated with me is that there is an even more primitive type of motivation than extrinsic: the drive to survive. (Or, more generally, physical and emotional needs.)

Under that hypothesis, the salary is there to take physical needs as a motivator off the table. If an employee has to worry about physical needs, there's no room at all for any other concern.

Once the salary is sufficient to no longer be a point of worry (will I afford my mortgage, can I support my spouse, etc) only then will additional money serve as extrinsic motivation.

(Which of course spells disaster for things like performance bonuses, which (a) are impossible to target at the right people (see Deming for more on that) and (b) literally designed to stop coming, reducing motivation overall.)