The metric system isn't hard.
What's hard is the fractional system... and even worse, trying to convert fractional measurements to the metric decimal.
Here's a demonstration...
If I'm working in metric, it's like holding coins in my hand...
If I say "give me 27 cents", then you could easily give me 27 pennies. Or a 25 cent piece and two pennies... or any combination of coins that make up 27 cents. The point is, it's not difficult. That's the metric system.
But if I say give me 13/16 of a dollar, then what? How much is it? How many pennies? That is the standard or fractional system of measurement.
Now I know exactly what 13/16" (.8125") is because I work in decimal equivalents all day long in my cabinet design software, but the average guy on the street doesn't even know how to read a tape measure, much less give you decimal equivalents, and he has even less chance of knowing how to convert it to metric.
Why we haven't gone to metric long since, I'll never know...
But just to let you know... 6 mm/ 25.4 = .2362.... in.
Or if you want to go the other direction, 5/16" = .3125" and .3125 * 25.4 = 7.9375 mm