> 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?
What good reason is there to believe a politician today could do better? Or are you simply trying to use slavery as a cudgel to get people to not argue with you? Politicians today are a ruling class that are worth hundreds times more than the average citizen, receive better benefits like healthcare, can use privileged information for financial gain, are often large land owners, etc. Nothing has changed - at all. If anything, it's probably gotten worse at scale.
> 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.
Political interpretation by the lunatics in a post-mcarthy world are the reason that people think the bill of rights should be flexible. "But the founding fathers never talked about the internet!" is a very pathetic excuse from a very pathetic ruling class that currently controls this country. Any violation of the freedom of speech no matter the medium is a deprivation of rights. We have chosen to interpret it differently. Once you start opening this can of worms up you get all sorts of problems with what is intentionality, what is association, does "free speech" include things I don't like, etc. Right now there are plenty of politicians on every side of the spectrum who'd love to ban everything they don't like.
What I guess I am saying is people are the problem. Greedy, slimy, humans. The world is no more complex at it's core than it was 200 odd years ago. We have made it complicated by spending time finding gotchas instead of treating the constitution as a series of unquestionable meta-laws. Fix the politicians. The document is fine. To use your analogy, allowing simple amendments would be like changing the entire PR review process because a handful of malicious engineers refuse to ever follow the rules. Just fire the engineers!