The Constitution itself recognizes its imperfections, leaving open the possibility for amendment.
As for the Declaration, I'm not sure much weight should be placed on its claims. It was written in what, one month's time?
First, it relies on "rights talk," a blunt instrument for any sort of political philosophizing. Second, it seems to change the Tory Divine Right of Kings to the Divine Right of the People. As with any sovereign, its high quality cannot always be presumed. A people, like a king, can become unfit to rule. The Declaration can keep that fearful possibility from even being considered, and so it seems to be a blind spot.
I think all of the founders understood that pure democracy and freedom aren't compatible.
"that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the Pursuit of Happiness -- That to secure these Rights, Governments are instituted among Men, deriving their just Powers from the Consent of the Governed."