I don't think the use of fractions was made obsolete by the metric system at all. The metric system certainly doesn't deprecate fractions. Its just that the ratio 180:100 can be reduced to 9/5 in your head. Dividing 9 by 5 to get a decimal requires pencil and paper.

The metric system is about weights and measures, not fractions. Half a kilo is just as legit as 500 grams.

Now try this on for size. 12 pence to a shilling, 20 shillings to a pound. Two shillings snd six pence is a half-crown (no such thing as a crown since the middle ages), and two shillings is a florin. Twenty one shillings is a guinea (for which no coin or note exists, although lots of things are priced in guineas).

Pennies are availlable in one, two, three, four and six denominations (penny, tuppence, thruppence, fourpence, and sixpence) and in half and quarter penny coins (ha'penny and farthing.

Now THAT's a system that really needed metric reform.