Yes, I get that, but that's what it will always remain — just a cope, but not much else.

The reason: Customary isn't criticized, as far as I can tell, based on how the individual units are defined. That wouldn't be really valid, because all base units are arbitrary, including those of SI.

No, the unit system is criticized as a whole. The way conversions are defined between each other, that's what makes them so much less practical. The fact that a foot isn't 10 inches, a mile isn't 1000 nor 10000 feet (hell, even 5000 would've been better). That 1 gallon of water doesn't weigh 1 pound.

In "daily life", both Celsius and Fahrenheit are mostly isolated scales – you don't relate them to other units very often, and you almost never have to convert them to anything else. So it doesn't really matter how they are defined — definitely not on the same level as length, mass or volume.