The Confederacy's Golden Circle ambitions also would not fit with an Union government that would likely develop expansionist desires itself. The CSA would not only have to deal with abolitionist raids, but it would be inviting itself into a second war with the United States over Cuba, Haiti, Puerto Rico, the Dominican Republican, Mexico and parts of northern South America. There would also be the Spanish to deal with. The entire theory of expanding southward was premised on expanding the slave power.
Britain probably wouldn't be too keen on Confederate expansionism, either, but unless Confederate attempts at expansion raise so much opposition that the Confederacy is conquered, this will not make slavery unsustainable.
What I do know is that restricting an entire group of people from being able to read (in violation of the First Amendment), restricting the right of an entire group of otherwise law-abiding persons to own firearms (in violation of the Second Amendment), forcing an entire group of people to accept Christ and reject their other God(s) (a violation of the First Amendment) are just a few extraordinary measures that slave states enacted to restrict the freedoms of blacks. The whole of African American society in the Southern United States just about lived under martial law, specially crafted just for them.
Before, during, and after the Civil War the majority of southern whites supported doing whatever it took to keep black people down and the majority of northern whites didn't care enough to try to stop them. This only started changing in the mid-20th century in OTL.
And a fraction of the population? 3.9 million slaves living in the United States in 1860, 4.4 million blacks total in the whole of the country. Mississippi's African American population today is 38%. Alabama's 31%. Louisiana nearly 39%. That ain't a small fraction of the states where slavery was most lucrative. The easy flow of information would not just allow for 50% of the 14% of African Americans to forever live in servitude.
What easy flow of information? Political and religious leaders in slaveholding states did their best to ensure that white southerners weren't exposed to abolitionist ideas. The mail was censored, presses were smashed, and men were lynched to keep southern white men from learning abolitionist ideas. Black people had the further handicap of it being illegal for them to learn to read.
This also does not take into consideration that a just few white planters owned most of the slaves. Income inequality and economic stratification on steroids, that's how one should describe slavery. The CSA would likely see an exodus of non-slaving owning and poor whites just because the economic system didn't include them.
To some extent that was already happening. In 1850 New York had more immigrants than the entire Confederacy would a decade later. For internal migration, roughly twice as many people moved to free states from slave states than moved to slave states from free states. OTOH, about 1/3rd of all families in Confederate states owned slaves, with many others profiting from slavery. Plus most southern whites feared that freeing the slaves would lead to white slaughter like had happened in Haiti.[/QUOTE]