My point in the earlier post was that slavery was assured labor not necessarily the cheapest.
Whatever their cost of death their loss would be a greater financial to the owner than to whoever employed Chinese coolies, peons, sweatshop workers in Northern factories and other cheap laborers showed the door when they unable to work any longer.
Well no, not really.
You forget that many of the slaves were inherited or bred at nominal cost to the owner and quite a few who were hired out were fully insured against loss.
You also overlook the slight cost of slave maintenance compared to free worker pay, the potential legal liability a northern employer faced if workers were killed or injured on the job, the need to compete for workers on the free market, especially when moving west was an option for so many (US salaries were among the highest in the world), and the existence of labor activism from a very early date.
Even imported Chinese railroad workers received relatively high wages compared to what you'd have to pay to rent a slave -- $24-31 a month (see link: http://web.stanford.edu/group/chineserailroad/cgi-bin/wordpress/faqs/ ), compared to $20-25 for a slave laborer. And that monthly cost came on top of the expense of bringing them across the Pacific, including insurance.
The relative cheapness of slave labor was one of the major reasons -- to get back to the point of this thread -- that the institution itself was in no danger of disappearing.