It demonstrated that they had the right ideals but were unable to put them into practice in one fell swoop. As Lincoln pointed out - the framework of the Declaration was an ideal to be pursued. The Founders' wisdom in what they put into motion necessitated the eventual end of slavery.
There's a big difference between moving toward the Declaration as an ideal as we still were in the time from the Founding until the early 1900s vs completely running away from those ideals as we have since.
Some of the Founders more than others. While many of them came from the North, where there were no slaves and slavery was generally unpopular, and some of the others like Washington and Jefferson held slaves but felt bad about it, there was a large enough contingent of Southern politicians who just plain favored slavery and had to be politically negotiated with. If you define "the Founders" as "everyone who attended the Continental Congresses or the Constitutional Convention", many of them were idealists, many of them had a vision, but many others were just ordinary politicians with their own agendas.
This also holds true for all American political history from the founding of the country to the Civil War.
It's also important to remember that this was before the industrial revolution, when almost the entire economy was agricultural, thus the South was far more powerful than the north.
The Civil War started right after the industrial revolution ended. By this time, the balance of power was greatly in favor of the North.
It's also worth pointing out that one of the early products of the industrial revolution--the cotton gin--made slavery far more cost-effective. It would have died out by natural economic causes otherwise.
Agreed. There will never be 100% admirable behavior in any person or political body.
The amazing thing about the Founders, though, was that the admirable forces were able to write the documents.
Compare that to today's "gang of eight" senators who created the immigration bill. That thing was a disaster before they had their first meeting. They had a unique chance to create solid immigration law, but have f'd it up beyond all reasoning.
The Founding documents were philosophical and political works of art. Where are works such as they coming form in today's society? Where are they being enacted into law?
There's a big difference between moving toward the Declaration as an ideal as we still were in the time from the Founding until the early 1900s vs completely running away from those ideals as we have since.