The economics of slavery were very favorable, as Eugene Genovese detailed in his "The Political Economy of Slavery". Up front costs were an investment, same as buying land or real estate. The value of a slave appreciated during their prime working years. Feeding the slave wasn't a big investment - just about all the food they ate was raised on the plantation itself. Clothing was rudimentary and housing wasn't a continuing cost, slave quarters were built only once. Genovese reports that the cost of maintaining a slave was nominal at best. In return the slave owner had a real asset that could be easily converted into cash and a predictable source of free labor.
Hire an immigrant off the boat and you'll avoid the up-front cost; in addition, if the pay is inadequate to keep the worker healthy and he becomes sick, no problem--just hire another worker off the boat.
You also don't have the return on your investment, the knowledge that the labor will be there when you need it, and most importantly the need to bid against others for that labor. Free labor tended to go where they made the most. The immigrants off the boat that you speak of weren't available in the South. The plantation owner would be in the position where he needed the labor at the same time everyone else did, and unless there was a surplus the cost of that labor was unpredictable.
The tougher issue with slavery, I expect, would be what to do with generations of people who had deliberately not been taught to exercise their own initiative. The Civil War didn't solve that problem; its effects persist 140 years later.
The toughest issue was what to do with all those black people, suddenly free and expecting to enjoy the same privileges as the white folk. That wasn't settled for 140 years either.
Was there a shortage of people wanting to come into this country?
I'll admit that there would have been substantial economic inertia, since among other things the farmers that had already bought slaves wouldn't want labor to be imported that would compromise the value of their assets, and immigrants wouldn't want to go where they were unwanted, but in time inertial effects will often be overcome by purely market forces.
What would slaves be used for in the winter months? Although I would expect they would be put to use operating cotton gins, I would think that much of the labor for those could be replaced with steam power if there were not a need to keep slaves occupied.
"I spent a few days near a large plantation in the country, whose owner had five hundred slaves; and I had free access to their huts… But though the slaves were not overtasked, they were provided with only a peck of corn a week. His overseer was ordered to procured coarse waled cloth enough to make each of them two garments a year. Hats and shoes were provided in winter for the wood-choppers and fence-builders, but for no others. The whole expense for food and clothing, reckoning the price of the corn and cloth at the market value, could not have exceeded ten dollars to each slave."
This was at a time when slaves were frequently rented out for well over $100/year, usually with food and clothing to be provided.