These days most people are aware that the Declaration of Independence was - by its time of publication - a heavily edited text. While I won't go into particulars I will note that some of the language which was in Jefferson's original draft concerned slavery. The fact that it was edited out (and the fact that the Constitution discussed and indeed even aided the institution of slavery) illustrates one of the fundamental problems which the "founding generation" failed to solve - the ideological, sectional, moral, etc. issues associated with slavery in a nation which declared that some sort of equality was a "self evident truth."
So I wonder whether their problem was ultimately a failure of imagination? What do I mean by this?
Well, before I make my argument I must first admit that the Continental Congress (the editors in other words) had no power to end slavery in the states - that sort of sovereignty rested with the states at the time. So whatever the editors of the Declaration thought about slavery, they could had no power to eradicate it on their own.
Still, there were plenty of examples in the classical world of a state in wartime freeing slaves who fought for a political body in dire straits (think of those Helots freed by military service to Sparta). A position which the young united States found itself in throughout the war for independence. Indeed many Latin American nations pursued such a strategy during the revolutions against Spain, and such emancipations rang the death knell for slavery in the former Spanish American colonies. So the editors could have made an appeal to slaves who wished to find freedom through service and while this would not have had the backing of law it was a solution well within the experience of most educated Americans at the time.
Then again perhaps such an appeal might have lost the young confederacy states like South Carolina or Georgia. Or perhaps the argument was never made (for all I know, it might have been - I am not terribly familiar with the discussion that surrounded the creation of the Declaration of Independence) because such an action - even though common enough in classical times - was outside the bounds of what their mental world allowed.