Do we need to call the fractional units as mills and bits? Why not stick with milli-bitcoins and micro-bitcoins?
People all over the world use the metric system. We should adopt it too.

Because people don't use full metric prefixes for money. No salesman puts a sticker price of 17 kilodollars on a car. No one says BillG is worth 40 gigadollars.
You might use an abbreviation and say a car cost 17 "k", just as you can say 17 "grand". It is a shorthand which is in common usage.
Mil has been used historically before e.g.

