You are making the same argument that Fredrick Engles made in 1847 in his "Principles of Communism" which served as the basis for the Communist Manifesto he and Marx produced a year later.
In what way do proletarians differ from slaves?The slave is sold once and for all; the proletarian must sell himself daily and hourly.
The individual slave, property of one master, is assured an existence, however miserable it may be, because of the master's interest. The individual proletarian, property as it were of the entire bourgeois class which buys his labor only when someone has need of it, has no secure existence. This existence is assured only to the class as a whole.
The slave is outside competition; the proletarian is in it and experiences all its vagaries.
The slave counts as a thing, not as a member of society. Thus, the slave can have a better existence than the proletarian, while the proletarian belongs to a higher stage of social development and, himself, stands on a higher social level than the slave.
The slave frees himself when, of all the relations of private property, he abolishes only the relation of slavery and thereby becomes a proletarian; the proletarian can free himself only by abolishing private property in general. The Principles of Communiusm. Fredrick Engles, 1847.
But to say this, Engles (and you) had to do three things. First, underestimate the effects of chattel slavery as practiced in the American south. Unlike other slavery practiced in the western world, chattel slavery by the 1840s did not simply reduce the slave to a position of legal servitude, it actually removed personhood from that individual. They were not recognized as people, but as property to be bought and sold with little or no recognition of any human rights or emotion on the part of the slave, or the master. It was a thoroughly inhuman system
Second, you must overstate the "poor conditions" of Northern factory workers. Their lot was not so bad as many believe, even the worst of it as simple logic would tell when looking at immigration patterns. If their life was worse than a slave, why then did so many save money to pay for more family and friends from Europe to join in those factories?
Fredrick Douglass wrote an interesting little essay on his experiences on the life of poor Northerners vs poor Southerners.
``Living in Baltimore as I had done for many years ... I had no proper idea of the wealth, refinement, enterprise, and high civilization of [the Northern section] of the country.... I came naturally to the conclusion that poverty must be the general condition of the people of the free states. A white man holding no slaves in the country from which I came, was usually an ignorant and poverty-stricken man.... Hence I supposed that since the non-slaveholders at the South were, as a class, ignorant, poor, and degraded, the non-slaveholders at the North must be in a similar condition. New Bedford, therefore, which at that time was in proportion to its population, really the richest city in the Union, took me greatly by surprise, in the evidences it gave of its solid wealth and grandeur. I found that even the laboring classes lived in better houses, that their houses were more elegantly furnished and were more abundantly supplied with conveniences and comforts, than the houses of many who owned slaves on the Eastern Shore of Maryland. This was true not only of the white people of that city....``I was not long in finding the cause of the difference, in these respects, between the people of the North and South. It was the superiority of educated mind over mere brute force.... On the wharves of New Bedford I received my first light. I saw there industry without bustle, labor without noise, toil--honest, earnest and exhaustive--without the whip. There was no loud singing or hallooing, as at the wharves of southern ports when ships were loading or unloading, no loud cursing or quarreling; everything went on as smoothly as well-oiled machinery. One of the first incidents which impressed me with the superior mental character of labor in the North over that of the South, was the manner of loading and unloading vessels. In a southern port twenty or thirty hands would be employed to do what five or six men, with the help of on ox, would do at the wharf in New Bedford. Main strength--human muscle--unassisted by intelligent skill, was slavery's method of labor. With a capital of about sixty dollars in the shape of a good-natured old ox attached to the end of a stout rope, New Bedford did the work of ten or twelve thousand dollars, represented in the bones and muscles of slaves, and did it far better. In a word, I found everything managed with a much more scrupulous regard to economy, both of men and things, time and strength, than in the country from which I had come. Instead of going a hundred yards to the spring, the maidservant had a well or pump at her elbow. The wood used for fuel was kept dry and snugly piled away from winter. Here were sinks, drains, self-shutting gates, pounding-barrels, washing-machines, wringing machines, and a hundred other contrivances for saving time and money.''
Source: http://members.tripod.com/~american_almanac/fredlinc.htm
The third thing you need to ignore is the fundamental incompatibility of slavery with the founding principles of this country. The founders attempted to deal with that incompatibility as best as they could at the time. Even under the Articles of Confederation, Congress banned slavery expansion into the western territories with the Northwest Ordinance. In the Constitutional Convention, the majority favored the gradual elimination of slavery, and most like Washington saw it as an economic inevitability since slavery in those days an increasingly less practicable system given the economy of the times. They were willing to compromise with the few deep south deligates on slavery because they did not think the institution could long survive. What they did not foresee was the growth of the Cotton empires of the deep south which by the 1820s took slavery from the brink of extinction, and turned it into the most profitable use of capital investment in the nation.
Thanks for the well researched response.