... which is arbitrary as well?

It's actually counting the atoms.

Sure, but the number of atoms which a mol represents is just as arbitrary as a kilogram or an ounce.

If you think like that, then everything is arbitrary. Even the rules of logic are "arbitrarily" chosen. Sometimes you have to fix some notions to gain anything at all.

Not exactly. Only for quanta that are not inherently countable.

Let's not lose track of what this tread is about, shall we? It's "what is larger, the number of stars in the milky way or the national debt?" One of those values is countable. How many stars? The other one isn't. How many debts? That question doesn't make sense. You need to introduce a unit, and the choice of that unit is arbitrary.