Yeah, the thing is that I actually don't terribly mind feet, cups, etc. as individual measurements. Taken alone, they feel intuitive. The first of two main issues is, as you mentioned, scale. And you could make the argument that you could just take the base units like feet, cups, etc. and decimalize them with prefixes. And that does alleviate a ton of problems with USC, but you also then run back into the issue of unit intercompatibility.

Metric units have quite an elegant and intuitive interplay that USC simply lacks. What's a liter? Why it's a cubic meter. But now if I try to relate USC volumes to cubic USC distance measures, things quickly fall apart. A gallon is 231 cubic inches exactly, which is a whole number, sure, but that's terrible for intution and for scaling. What's a kilogram? Why it's the mass of a liter of water. If you try relating pounds to units of volume, you might settle on one pound per pint, but that isn't true, as it's actually 1.041 pounds. So while it works at that scale, it quickly begins to fall apart and becomes completely inintuitive.

So unfortunately, even if we introduced intra-unit scaling by choosing one base unit and scaling that, working with USC would still be a nightmare when trying to intuit between different units. And this is, of course, not including things like horsepower that may have been intuitive 150 years ago but now are almost exclusively used 1) at minimum in the hundreds and 2) by people who have literally no concept of how much a horse can turn a mill wheel.

And it's sucks if you want any kind of precision. What's half of 15 5/8? Fuck it, I'll just use centimeters.