Constitutions are changed often based on a majority vote.
Not with a simple majority of course, usually it's much harder to change a constitution.
(or, again, it falls back on the "it's not really a constitution" clause, as hinted before)
yeah, constitutions worked marvels all the time everywhere as anyone can see.
it's just a matter of scale: the bigger the political unit and the bigger the troubles that it procures to its subjects in every time and place.
Luckily what is not sustainable won't be sustained for long, since if all depended just on the wisdom of the sheeple we would be really fuck'd for eternity.
Sure, I wasn't implying they work, they don't, I was just stating that unless they do, you can't call it Democracy.
BTW I fear you're too optimistinc about not being sustainable, I fear that they will manage to enslave everyone indefinitely unless they are overthrown.