Critics of the metric system often cloud the issue by saying things such as: “Instead of buying 1 1b of butter you will have to ask for 0.454 kg of butter.” The implication is that life would be more complicated. How might you refute this?

The SI unit of mass is kilogram (kg). In British unit, the unit of mass if pound (lb).
The relation between kilogram and pound is
1 1b = 0.456 kg
Thus, buying of1 lb of butter is just equivalent to buying 0.456 kg of butter. It will not change the amount of butter bought. The only thing is that it depends on which countries are you buying butter. Different countries have different metric system for measuring units.
Therefore, implication of different metric system will not make life more complicated.