And in some cases, using metric would simply be worse in daily life.

No?

Like using Kelvin instead of Celcius

They are the same thing. They just use different starting points.

meters per second instead of kilometers per hour

Meters per second and kilometers per hour are both the same system, called the metric system.

seconds in general instead of hours and days

What are you on about? Seconds, hours and days are all part of the same system.

Fun fact: The fuel consumption of a car that needs 10 litres / 100 km (24 miles per gallon) could be simplified to 0.1 mm2

Actually yes. 10 L = 10 dm^3 = 10 000 cm^3. That means 10 L / 100 km = 10 000 cm^3 / 100 km.

To simplify further:

10 000 cm^3 / 100 km =

10 cm^3 / 100 m =

0.1 cm^3 / 100 cm =

0.001 cm^3 / 1 cm =

0.001 cm^2 =

0.1 mm^2

Is it metric? Yes. Is it practical? Not really.

It was already metric from the start before any mathematical simplification was done, so the metric system was definitely practical here.

Is the simplest mathematical form always the same as the simplest practical form? Definitely not, but that has nothing to do with the metric system.