I thought this is was why the SI was so superior to the english system, at least that is what we were told.
Really? Who told you that SI was superior to Imperial because the replicas of the International Prototype Kilogram were exact and invariant?
And are you aware that since 1893, the definition of the avoirdupois pound (at least in the United States) has been in terms of the kilogram?
You were told that SI is superior to Imperial because it is.
My hunch is the universal gravitational constant isn’t.
Do tell. And does your hunch have a hypothesis to explain why all the replicas of Le Grande K have varied by different amounts? And are you aware that masses are measured using gravity-independent means?
The fact that the kilogram is defined as the mass of an artifact is a relic, left over from the 19th century when representing standards with physical objects was the best that could be done. Soon enough, this will become a nonissue when the kilogram is redefined by fixing Planck's constant, just as the meter was redefined by fixing the speed of light in a vacuum.