But metric measurements are based on an estimate of the world's circumference made in the 1790s! What sort of basis is that, I ask you!

Metric's units may be arbitrary, but they're far easier to use. In my field (real estate), can you imagine the difficulty of calculating with these units: 1 acre = 43,560 sq.ft. = 1/640 of a square mile?

The old system of rods and chains was easier (10 sq. chains = 10 acres), but I've never seen in used in documents later than about 1910.