The constitution would not work if had a fixed meaning based upon the words because words often have different meaning to different people under different circumstances. That's why we have Supreme Court. If SCOTUS rules against the popular will, then the people can amend the constitution to overule the SCOTUS; and if SCOTUS misinterprets congressional intent, then congress can pass remedial legislation to correct the error.
The Free Speech clause of the 1st Amendment is a good example. The words are straight forward and without exception, yet SCOTUS has drawn many exceptions over the years by going beyond the plain meaning of the words. If the SCOTUS is irrelevant as you suggest, then we really need only two branches of federal government. And if SCOTUS can't look beyond the precise words in the text, then it has no basis for the "fighting words" and national security" exceptions to the free speech clause.