[Zhang Fei]

The South could split the North today and no one would do anything about it

I'd say the main reason is smaller families. The average household has 2 children. Presumably one of those is a son. No way any Union government could bear the 500K dead only sons it would take to hold the country together against a determined secession effort by the red states.

[kidd]

I don’t think there would even be a war.

I think the South+Midwest could escape
gay marriage
Obamacare
creeping socialism

by just deciding to walk away and without a shot being fired. Who’s going to stop the South+Midweat from becoming the better America?

[x]

I'd say the main reason is smaller families. The average household has 2 children. Presumably one of those is a son. No way any Union government could bear the 500K dead only sons it would take to hold the country together against a determined secession effort by the red states.

The same logic works the other way around: no way is there going to be any "determined secession effort" by anyone. Other reasons are that we have been such a rich country -- and so preoccupied by mass amusements -- that no region is going to get together to tear the country apart for ideological reasons. And no issue is as monolithically divisive and as impassioned as slavery was a century and a half back. For better or worse, we're still one country.