The system is embedded in standards, such as SAE. It is embedded in industries such as construction. It is embedded into statutes, regulations, and deeds for real property.
I knew a civil engineer who first worked in the private industry and later got a job at the DOT.
At the construction company, they did everything in metric internally. Once a design was finalized, they would convert everything to imperial and submit to the DOT. DOT is behind the times, she thought.
When she got to the DOT, she learned their procedure. Before engineers got started reading the plans, a secretary would translate units from imperial to metric for internal use.
The fact that an old contract specifies units of rods and farthings is not an argument for anything other than defining official conversion factors between the old way and the new way.