In both cases, you have fractions ...(2 2/3 and 5 1/3)

That's just my point. Continued exposure to the metric system removes the though process of fractions other than tenths, and a person brought up using the metric system would not have been able to give the above answer.

Years ago I was working on a conveyor system that had been designed in Germany. It was clear from the designs that the engineers were incpable of even thinking of dividing by three.

They had conveyors with one central leg, or three, or five central legs, but not one conveyor with two central legs because that would have divided the length into three; and they just couldn't do that, even though, in many cases, that would have been the obvious solution to an English designer.

Any restricted denominator system is seriously limited and to make such a system the norm for every form of measurement is short sighted indeed.

Decimal fractions can be very useful, engineers use them often when working with inches. There is nothing stopping anyone using them whenever they are more convenient, but why force them onto every form of measure?