sure, but at that point you can just use any arbitrary unit if all you need to do is keep it in a rough ratio. the SI system is useful because it's standardized, allows for precision, and you can easily convert units. A liter is a decimeter cubed, try doing that with feet and gallons.

but yeah, i agree people would benefit from more straightedge and compass construction skills instead of trying to read the milimeters of a measuring tape that's not even guaranteed to be straight