That's the 'heads' side. The 'tails' side says, "How could the men who wrote it know what would be needed in the 21st. century?"
I like what you said about the constitution being the document whereby the states define what powers and responsibilities they'll give the federal government. That makes sense and should be unambiguous. Looking around here, though, there isn't a social, legal, political or moral issue that doesn't need to be examined constitutionally. In that sense, it looks like a ball and chain.
the men who wrote it make it so it could be changed, but the process is not an easy one, and there is a reason for that...the founders wanted long drawn out debate on any changes to the constitution, so that all sides of the situation would have their say.
under the articles of confederation the states themselves where their own masters and could do just about anything, the central government was weak it could not tax and it could not solve problems between the states.
the states being their own masters fought each other over trade and created their own problems, like creating inflation because states printed their own money, they each had their own armies and did not work with other states in defense.
to founders intended to fix the articles, but instead wrote a new constitution, fixing the problems the articles with that new constitution.
the states gave some of their powers to the new federal government like printing money, the military, and the congress was given power to regulate commerce among the states to keep states from fighting each other, but congress was not granted power to regulate inside of the states or over the people, the states retained that power.
what the founders wrote about the constitution and what it means can be found in the federalist papers, what an article, section and clause meant in the 18th century, should mean the same thing in 21h century.
but today you have people who want to change the constitution but cannot get an amendment so they try to reinterpret the constitution to allow them to do things the constitution does not authorize.