I _very much disagree_ with the assertion that the US constitution's rigidity is its greatest strength. Similar to taking on too much technical debt, this rigidity makes it impossible to update in response to changing circumstances! I'm not convinced that's ever a good thing; what good reason is there to believe that a constitution conceived over 200 years ago (when the US had roughly 1% the people, ~1/3 the land, legal slavery, an early-industrial / agrarian economy, etc., etc., etc.) is useful today?
Even if we agree that "free speech" and "free association" are reasonable universal rights (and I think we do), what these concepts mean in practice must necessarily evolve as technology gives us new ways of speaking and associating - and we will _always_ have to deal with the difficult question of what to do when someone, through exercising these rights, deprives others (intentionally or unintentionally) of the ability to exercise theirs. This balance necessarily changes as our methods of exercising free speech and association change.
Overall, I'd argue that this rigidity - in the constitution, in the calcified two-party system, in US politics / political systems in general - maybe started out as a unique strength, but is _very much_ a weakness in more complex and rapidly changing times.
[1] https://en.wikipedia.org/wiki/Amendments_to_the_Constitution...