This is simply not true. If anything the U.S. was less "isolated" at the time of the founding that it is now. It was surrounded on all sides by hostile or potentially hostile powers throughout most of the nineteenth century including possessions of the British Empire, the Spanish Empire, and the French.
Isn't this excactly what the libs say: medicine has gotten much more complex since the time of the founders, transportation has gotten much more complex since the time of the founders, education has gotten much more compmlex since the time of the founders, citis have become much denser and the second ammendment no longer makes senese like it did in the time of the founders, etc.