Everything is an arbitrary division when we get down to it. Doing away with states would require a complete rewrite of the constitution, and a fundamental shift to the country as a whole. I personally like the Republic concept and ability for states to experiment with things that might not be popular or a priority for the entire country. This will have good and bad outcomes on these experiments, but it’s how we have things like decriminalization, universal healthcare attempts, etc. Without the “all other things not innumerated belong to the states” this isn’t possible, and removing state representation removes that.