There is a problem here. You're saying that :
A) The Declaration of Independence contained a set of ideals that are good and moral to this day.
B) That these morals were foundational to the USA, and thus part of the core of the nation.
Now, we can assume that if those ideals were there, and that they were foundational to the USA, that the revolutionaries, the guys who took a world power to fight for independence, would have actively believed in them and tried to execute them, thus leading us to point
C) Revolutionaries try to fight for ideals.
The question then is, how did we get to point
D) Slavery exists without excessive controversy for a long time.
The answer is that it can't. You can not claim that the US was founded on the "ideal of all men being born equal" while the nation clearly believed that some men were less equal than others. So, point B fails. Whatever good ideals the DOI may have had, it is clear that they were not held by the founders and people of the early US.