There's nothing inherently more natural about cooking in the metric system, people just prefer base 10 these days. People balk at 4 quarts to an arbitrary gallon but love 1 liter being the arbitrary cubic volume of 10 ten-millionths of the distance from the Equator to the North Pole passing through Paris [but not quite].

Cooking by volume was natural before everyone had accurate kitchen scales. You didn't have a digital tare button in the 1800s but you did have a bunch of containers in common sizes.

what happen when you need 3/4 of a cup ? Or 1.5 cup ?

Generally you have 4 sizes: 1, 1/2, 1/3, 1/4. You just use a combination of 2 sizes (1+1/2) or multiples of your smallest size (3x1/4).

You usually don't need high precision for cooking, common ratios are good approximations (1:1, 1:2, 1:8, etc...). Baking is a different beast and I don't know how people did it before weight.

Also, fuck tablespoons and teaspoons. They should just be replaced with 1/16 cup and 1/32 cup.