> What if anything is smaller than an inch?

Then we use Pica and Points. You know, like 12-point font. Which is 1/6th of an inch tall. 72-points to an inch, 12-points to a pica.

Get with the program man!

Too bad we had to go and make displays with different pixel densities...

Most people never use units smaller than can be accounted for by fractional inches and ounces in their daily lives, and never did. The only people who ever used grains were pharmacists, jewelers, and the like. The only people who ever had to resort to thous were precision machinists and the like.

Open Amazon, and search for something as mundane as plastic bags or trash cans. Notice that their thickness is specified in "mils". How many people make purchasing decisions on those products, for use in their daily lives?

All you need to know there is that a 3 mil is thicker than a 2 mil. That doesn't require any kind of calculation at all.

Most units are used in the same manner - no-one is mentally converting kilometers into meters to compare two km distances, for example.

The point is that people do have a need for units that are tiny fractions of an inch, even in day to day life.

Miles and km are the same, just on the the other end of the scale. A straight magnitude comparison requires no calculation.

The units that people need to perform calculations with are the ones used in measuring for cooking, sewing, home improvement projects, and things of that nature.