The founders came from England, which has the world's longest unbroken political tradition (apart from 11 years during the English Civil War). England has top-level cabinet positions that were established 800 years ago. So I doubt the founders would be surprised that their constitution was still in use 236 years later.
Regardless, what the founders believed is relevant because they're the ones that wrote the currently operative legal document that governs the country. We can replace that document whenever we want! But until we do that, the document, and what its authors intended it to mean, are binding on us.
Along this line of thinking, surely there’s an unbroken administrative / bureaucratic tradition running China that spans multiple royal dynasties and perhaps even the recent ideological upheaval. Can we call that an enduring government?
Regardless, what the founders believed is relevant because they're the ones that wrote the currently operative legal document that governs the country. We can replace that document whenever we want! But until we do that, the document, and what its authors intended it to mean, are binding on us.