International System of Units overhauled in historic vote

My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly.

Edit: I think your question boils down to "Why do we have two separate units for mass, u and kg, connected by the Avogadro constant?" Most answers dismiss your original question as Avogadro constant is not a unit. But u is a unit and it's the point why we have this constant.

Edit2: To further emphasize my point look at the mass of neutron[1]. It's listed both in kg and u. Note the number of decimal places.

[1] https://en.wikipedia.org/wiki/Neutron

I don't know about trade, but kilo standards were deviating by tens of micrograms from eachother, some things are sold at prices which warrant accounting for those micrograms, so a completely stable international definition seems helpful.

Certain types of scientific instrument will need to be recalibrated to meet the new definitions.

Has a "metric" version of time ever been proposed? Seems odd to me that we're "ok" with divisions of 12 for time but not length.

I guess for science seconds are basically the only measure of time that usually matters?

Is there any compelling reason why the US should switch to the metric system for daily use?

Metric is already used universally in science and engineering, which seem to be the main fields in which consistency across countries matters. I completely agree that in physics, chemistry, engineering, medicine, etc., it is important to use standard units. But what harm is it doing in practice if in everyday life, Americans say things like "I weigh 180 lbs" instead of 81 kg?

The argument that the metric system makes unit conversions easier (e.g., the fact that it's immediately obvious how many meters are in 20 km, but not how many feet are in 12 miles) is true, but not very compelling to me. I virtually never find myself wanting to do these conversions in a non-scientific or non-engineering context, and on the very rare occasions where I need to, I can always look them up on Google.

A good comparison is degrees Celsius, which are just as arbitrary/unscientific (the standard metric unit is the Kelvin). Somehow people around the world get along fine using Celsius.

I read the uk address and was afraid they had voted to leave the metric system, as some sort of Brexit associated madness.