I can see that. I've thought a great deal about that two. On the one hand, societies and cultures evolve, so it's important for governments and institutions to evolve with them. I think building that concept into the framework itself would be important.
On the other hand, it comes with a different set of challenges. Of the two most obvious that stand out to me, one is ideological and one is practical.
On ideological grounds, the point of a government charter such as a constitution is to create a stable system based on certain principles that are at least in theory permanent, inherent, or universal. In modern times, often that includes provisions for certain immutable human rights.
If we make constitutions more malleable, then a side effect of that would be that the rule of law also becomes more flexible. How can we guarantee that over time, in a century or two or even longer, those human rights might gradually become more mutable, until they eventually start to disappear altogether?
On practical grounds, it comes down to how the changes themselves would be implemented. Would the constitutions be updated by citizen's referendum, by a constitutional convention of state/provincial governors and/or legislatures, by federal-level legislatures? How could that work? And how could the courts provide oversight to such changes if their key role is to ensure the other branches are following constitutional law?
It seems the opportunity for a constitutional convention is rare, because according to the safeguards that are rightfully built into said constitutions, changes require widespread support. That isn't guaranteed to occur every few generations, nor even every century. And do we really want to impose some rigid schedule for changes and watch politics degrade into a hectic battle for a supermajority every few dozen years?
Furthermore, what if in a century or two, a new wave of fascism happens to take a supermajority and enough state/provincial governments to hold a constitutional convention? That would be like a reversal of the intention, the dark flipside parallel to the bright promise of a governmental system that can adapt to the changing needs of the times. There's no guarantee that it would be a progressive government in power when it comes time to make updates to the fundamental structure and function of government.
So in that respect, I think stability is important, and some parts of government/constitutional law should be immutable.
Maybe a layered charter could provide a good solution. Some parts that are permanent and can never be changed, and some parts that must be updated on a schedule; say at the turn of each new century.
The challenge then remains in deciding, from a present standpoint, what aspects are enduring and universal, and what aspects are relevant to time-and-place.