No, there is no must per se. There is only a should in order to ensure an outcome, and as you stated, your intent is to retain the capacity of self-legislation.
Another question that is hiding in yours is, "Must any freedom be guaranteed?". If so, where does the necessity derive from? And how are conflicts of certain freedoms be solved, e.g. when people use free speech to agitate or spread lies?
In the end, it's about values and how those values are prioritized. In Germany for example human dignity takes precedence over freedom of speech and therefore among other reasons Holocaust denial is considered a crime.
A constitution reflects values of a society at the time when it was written. It does not ensure that those values remain the same over time. And there may be a time when people will value other things more than freedom of speech and maybe they even want to give up the capacity of self-legislation. You cannot stop that. And then a constitution will be changed.
And even if the people would still value those things, you cannot ensure that a political system will remain in place with the help of a text.
A powerful minority may use loopholes or start a revolution and proclaim their own set of rules.