Well, it needs 2/3 of Congress to propose it, and then 3/4 of the states to ratify it, so it's a bit harder than that, but your point stands. The constitution does indeed have provisions to change with the times, provided the nation agrees.

Stability is also important of course, but a reasonable balance seems to have been struck. The constitution does indeed evolve with the times, while still remaining more or less the same document, upholding the same basic ideals.