Lot of assumptions here.
Yes, AutoCAD is essentially unit-agnostic, but if you create a drawing and you’re thinking in inches, then you’re going to have to scale by 2.54 / 25.4 / 254 / 2540 / 25400 or whatever to get the appropriate metric dimensions, and by 1/2.54 or w/e to get back to inches.
However, the world has moved on since then. I’m using Inventor and ReCap, which absolutely do care about units. My default units are set to inches for both.
Let’s look at a workflow case study.
This housing is a part I needed to reproduce, because it’s broken:
Removing the shaft and two gears, I scanned it, and got this:
(note: it’s already been into Cloud Compare and had the RGB value per point changed to a scalar field based on Z height, and it’s been into Meshlab to get aligned on planes. The red circle shows where my scanned part was broken.)
This point cloud gets imported into Autodesk ReCap so that it can be saved into the format that Inventor wants to see. As I stated, my default units in ReCap are set to inches. ReCap says “oh, damn, this is not inches,” and automatically sets units to meters instead. (It assumes meters rather than millimeters due to the density of the point cloud, which is pretty insane right now.)
Obviously it’s not meters- this part is 2-3 inches long.
Set it as overlay in Inventor–again, in a native-inch file–and Inventor recognizes that this is still a metric file, and we scale by .001 to get mm vice meters.
Next we open up the model of the part I’d previously made based on calipers and micrometer and compare the two. (Note- I also machined this part previously, and it’s pretty nearly a perfect drop-in fit, though a few areas that I’d simplified from the original part needed to be modified slightly.)
Looks pretty close… let’s overlay them directly.
If the original file was not created by Handyscan such that the location distance between points was meant to be understood using SI units, then those holes would not line up.