> effectively impossible

Let’s assume you are correct. It is impossible to ever make this change for reasons X, Y, and Z.

What happens when stores just can’t get pennies anymore? Does the sky fall?

There are many examples of these types of jurisdictional conflicts in the US due to the strong decentralization of authority. These situations almost always rely on non-enforcement, which works until it doesn't and then you find yourself in court. Enforcement sporadically happens, or more commonly happens, someone with a personal axe to grind demands enforcement happen.

I'm all for eliminating the penny and rounding to the nearest 5 cents or whatever. But I am not so callous as to ignore the reality that doing this de facto forces many businesses to break the law because compliance is impossible for reasons completely outside their control.

Maybe you're cool with breaking some eggs to make an omelette but I find it pretty gross and immoral to dictate change without satisfying the preconditions that allow it to occur legally for everyone involved.