Based on international law the states would maintain their current borders, that means the urban areas would come along for the ride (IF under an actual referendum the majority was to vote in favor of separation vs the non democratic exercise that is US elections)
The movement would probably see the creation of a new Union instead of just having a bunch of small new countries, so it would be rich enough to equip itself and create an army (and those voting in favor would probably jump on the occasion to defend their new country).
Separation doesn't happen overnight, you go and fetch support from other countries so you're not left without any allies or international recognition if the vote is in favor (France was ready to recognize Quebec if any of the two referendums had been in favor of independence).
You didn't create a "best case scenario", you just created a scenario that fits your opinion on the subject.
Disclaimer: Am not from the USA, would gladly see it getting split in multiple countries just like I would gladly see Canada split in multiple countries as I think in both cases it would stop some parts of the country from slowing down progress in other parts. Ex.: If Mississippi and its citizens want to live in a third world country so much then so be it, let the rest of the US move forward.