All the money that was previously wasted paying the endless shitpile of state level politicians...

Which, by the way, is next to nothing.

...can then go toward building roads, bridges, fire and police stations and staff for said stations, primary schools, and social safety-net programs to encourage the unemployed to strike out on their own as entrepreneurs.

I don't think the Federal government should be building infrastructure or providing public services for a local jurisdiction. I think the states definitely know better where they need a road built, or where they need a bridge built, or which of their cities desperately needs a better fire station, or more police officers. The Federal government already has it's hands in building infrastructure for the states in the form of earmarks, which are colossal wastes of money. It'd be better if the projects that earmark money were set aside for were simply given to the states -- or better yet, not taken from the people in the first place.

Another plus is that single unified nation would be much easier to manage and change the laws of than 50 non-sovereign states.

Marijuana has been illegal for 70 years, based on no solid science whatsoever -- at the Federal level. The Federal government made the decision to invade Iraq. The Federal government failed to shut down BP before the Deepwater Horizon catastrophe. I used to think that we ought to have a single government as well, and I used to be a proponent of a single, global government -- but now I'm vehemently against both ideas. Let countries and states make their successes, so that the countries and states that make mistakes can emulate them. I'd argue that, overall, such a system would be the quickest vehicle for effective change.