The problem is, if you rely on tools - and a constitution is nothing else than a tool - you will end up developing a reliance on that tool. And all of a sudden, that tool breaks, and you're fucked.

Democracy must be fought for every single day, and for that matter so does all progress. Democracy isn't something a society automatically converges to and which everyone loves, it must actively be maintained or someone will come, sow mistrust and preach "easy" solutions - and then you wake up and your country isn't a democracy any more.