I find it very difficult to see slavery lasting into the 20th century. Remember that the southern slave economy was only profitable for slave owners who grew cash crops, like sugar, cotton and tobacco. A big reason slavery died out elsewhere was there was no need for slavery, or it wasn't profitable to use in the crops or business.
Really? Can you name a single country in the entire world where slavery was ended because it was unprofitable?
Slavery was ended for moral and political reasons, not economic ones.
Contrary to what some people might believe, slaves are not cheap.
Indeed they're not. But neither are free workers.
They cost money to purchase, house, clothe, and feed.
So do free workers, via these things called "wages". What happens with a free workers is that you pay them enough "wages" to allow the free worker to pay for their housing, clothing, and food etc. If you don't give the free worker enough "wages" to do these things, then the free worker won't work for you.
In fact, unless there's a labour surplus, you have to pay a free worker
more than enough for housing, clothing and food, because the free workers wants to have money to spend on things other than the bare essentials of shelther, clothing and food. And if you don't pay the free worker more than that, someone else will.
With slaves, the cost of food, clothing and housing is provided directly by the slaveowner, but if you pay a free worker, the cost of that same food, clothing and housing is built in to what you pay them.
By the time the Civil War came about, it was becoming cheaper to pay a white immigrant to work horrendous hours in equally horrendous conditions in factories and mines than having a slave do the same job, because factory owners didn't have to house, feed and clothe the workers.
This happened in a few cases, but the reason had nothing to do with the cost of housing/food/clothing. It was because white immigrants were expendable. No-one cared what happened to them, in the pre-workplace safety era. So they got some of the really mucky jobs - e.g. working on dikes - in places where there was immigrant labour available. (Baltimore and New Orleans, mostly). Native-born whites, however, were rather more expensive than immigrants, and also rather more averse to taking the really dangerous jobs.
Even then, in many cases the switch to free immigrant labour went back to slave labour, because the immigrant labour was seen as unreliable. Particularly in terms of striking, quitting with no notice, or showing up to work drunk.
If a worker quit, or got killed or crippled, then a replacement could easily be hired. Slave owners, however, had to pay for their labor up front and over time. If a slave got injured, then that was money down the drain because the owner had to pay for medical treatment (if he gave said slave any) and an injured slave cannot work as well as a healthy slave.
Slaves could be - and were - insured. Outside of the really, really dangerous jobs - and not always then - slaves were still preferred, and demonstrably cheaper than the alternatives.
To take a couple of not-quite random examples (from Starobin): in 1855, the Virginia and Tennessee Railroad reported that slave labour cost $11 monthly while free labour cost $40 to $50 monthly. A Kentucky hemp manufacturer who switched from free labour to slave labour reported that he reduced his costs by 33%.
On the idea of expanding slavery across the country, remember that the crops grown using slavery can deplete the soil rapidly.
Whether the crops are grown by slaves or free labour doesn't make any difference to whether those crops exhaust the soil. Intensive plantation agriculture does that, no matter whether slave or free are doing the harvesting.
What happened in the OTL western expansion of slavery was that it was simply cheaper to move west and use new lands than it was to use fertilisers and other methods to improve the soil.
That exit valve only applied as long as there was plenty of good cheap land out west, though. It was starting to run out even before the ACW. In the older areas, especially Virginia, planters were starting to turn to animal manures, crop rotation etc to restore the most exhausted soils. (Edmund Ruffin was particularly notable in that regard).
The Confederacy had hoped to expand into what’s now Arizona and New Mexico, but that area is not ideal for large scale farming. Even if you rotated crops to replenish the soil, that’s money out of your pocket for that growing season.
True as far as it goes, but the need to rotate crops/buy fertiliser will come back to bite the new cotton belt areas regardless of whether slavery expands further.
The bottom line is, no one will buy a slave worker if given a cheaper alternative.
True, but the reality is that in virtually all cases, the slaves were cheaper. The only real exception is jobs where the risk of death was so high that it was better to use immigrants. But that was only a rather small percentage of the jobs.