One of the benefits of the metric system -- versus the imperial system -- is that you have fewer conversions. Rather than having inches, feet, miles, furlongs, rods, hectares, acres, gallons, fluid ounces, teaspoons, and so forth you have meters. Then you can have centimeters, kilometers, square meters, cubic centimeters etc., but you can convert amongst them trivially.
Well, YOU can, because you're very comfortable with using base 10. But the appendage comment enough points out this is a peculiarity to members of our species, rooted in cultures which use mathematical systems which eventually owe something back to the fact that a recessive trait is near-ubiquitous.
Yes, polydactylism is dominant.
I... I'm not sure how polydactyly being a dominant trait matters at all here. Having ten digits may be recessive, but so is not being a dwarf. Meanwhile, pentadactyly, despite being recessive, traces back to ancestral tetrapods. Having ten fingers is the basal state.
The temperature scale didn't have that issue. These days, the main advantage of the Celsius scale is its near universal ubiquity (which is a significant advantage, by the way!).
Well, except that your grams, metres, and litres aren't entirely arbitrary in their definitions. I mean, one of them is, but the others are based on having water having a density of 1 and handy conversions between the units (1 ml = 1 cm^3) under "standard atmospheric Earth conditions" (another way that the system is Earth-centric). Celsius scale has this advantage as well - 0 and 100 are special for water in such conditions. Actually, it's somewhat strange to my mind that the figures work out on round degrees in Fahrenheit. Probably if I looked a bit more into the history, I'd find that there is a reason for this (like it got rounded or changed at some point).
And yes, this is another thread ne-ne-ne-ne-necro.
Yes, obviously all of the original metric definitions are Earth-centric, which seems entirely reasonable given our lack of contact with extraterrestrial lifeforms. It's not exactly a cultural ethnocentrism that causes systemic discrimination.
And none of the original metric units were arbitrary that I know of. The kilometer was one ten-thousandth the distance from pole to equator; the kilogram was of course based on the mass of a liter of water, with liters derived from meters. The second was based on its traditional definition, and you've already noted how the Celsius (and therefore Kelvin) scale was derived. I suppose the ampere, candela, and mole are not Earth-centric, except insofar as the first two rely on the definitions of the meter.