No, changing social policies is completely unhistorical and does not add anything to gameplay!
For the historical part:
It's not like people throughout history ever changed their views on anything. All societies that used to have aristocracy or other right-by-blood laws, or who used to hold slaves or considering their leader to be a son of god still have aristocracy, slavery and divine rulers.
And for the gameplay part:
Adding more choices to the player just makes the game too confusing. You should need to pass kindergarten to be able to play Civilization.
It's true! Nothing has ever really changed in any nation or culture over thousands of years, so it only makes sense that Social Policies are rigid and fixed for the entire game. And as you so rightly point out, giving players options and choices in their games is only going to make things more difficult and confusing, and I for one don't want to bother with that sort of headache-inducing thought process.