Radians are not metric units.
Degrees are not US Customary units.
Both work with yards/feet/inches/etc
Both work with meters/cm/mm/etc
Neither is a sYsTEm.
Stop thinking about the resulting vertical inches or centimeters downrange. It means nothing.
The radian is the SI unit for angle. The degree of arc is the imperial unit of angular measure. No amount of stating otherwise changes that. The fact that they are dimensionless does not change that. The fact that you can mix the two systems does not change that. I can measure something in feet and weigh it in grams, but that does not change that one unit is metric and the other imperial.
Now, if you want to argue that mixing the two systems does not matter, that is a more supportable position.
A radian is defined as a unit of angle equal to an angle at the center of a circle whose arc is equal in length to the radius. This conveniently gives pretty simple conversions using a decimal unit of measurement. Part of what makes it more ergonomic.
An MOA is 1/60th of a degree, a degree of arc is just 1/360th of a full circle. I'm not aware of an intuitive way to make the system work with trigonometry. I am open to being educated as like most Americans I learned the imperial system for angular measure first and my brain still defaults to it.
What ever a "sYsTEm is, is not what I'm talking about, but both are a system for measuring angles. Again, pretty basic definition.
The fundamentals is the geometry, the resulting change in vertical (and horizontal) distance down range is the entire point. Yes, it is theoretically simpler and better to talk in angles rather than doing trigonometry, but this does not work well in many situations (how many MOA for the vitals on a deer? Oh, wait, by definition you cannot do that.) It does work great if everyone one has a ranging reticle using the same angular unit of measure.
1 mil subtends 1 foot at 1,000 feet
1 mil subtends 1 meter at 1,000 meters
1 mil subtends 1 inch at 1,000 inches
That said, I still think the mils Quick Wind is simpler. I'll note that Accuracy 1st doesn't have a MOA QuickWind solution for Kestrel yet.
Yes, the radian is a super unit of angular measure, as you so nicely demonstrate. Now, as it is how we use the system that makes it ergonomic, is your rangefinder set to measure in feet? Do you move POI 1 inch or 1/10th of a foot?