Why should literally anyone in the 21st Century give a ◊◊◊◊ what they thought? Why is it held as nearly sacred and not a legal document to be amended when society gains more information or changes?
It seems like there are a few misunderstandings here.
The US Constitution is well-understood to be a legal document that can be amended as society changes. In fact, it's been amended several times. You should already know this.
That said, when it comes to Constitionally-enumerated rights, there's very little interest in amending the Constitution to get rid of or modify a right. Rather, what the courts have done is to modify how the clause in question is interpreted. For example, there's no interest in repealing the right to bear arms, enumerated in the second amendment. Instead, as society changes, the courts have modified the interpretation of what a "right to bear arms" means. Similar things have been done with regard to many of the other rights enumerated in the Constitution.
Not only that, but the courts have established that while these rights are sacrosanct in principle, some amount of infringement may be necessary from time to time. Thus the right to free speech is limited. The right to bear arms is limited. Etc.
In practice, therefore, the Constitution is not a sacred work set in stone. It's just that we find it more practical to modify it through interpretation, rather than amendment, whenever possible.
But rights are still rights. You can't interpret a right entirely out of existence. On the other hand, we don't actually interpret the second amendment (or any other part of the Constitution) entirely on what we believe the authors meant. We take that into account, certainly, but we also take into account contemporary practicalities and considerations. That's how we end up with the relatively short list of arms that we have a right to bear.
In conclusion, you're wrong. The Constitution is very much seen as a legal document to be amended, and more importantly, to be re-interpreted over time. The application of the second amendment has been modified repeatedly by the courts over the years.
What we hold sacred is not the document, but the rights it enumerates, and the principle of rule of law.
What amendment would you suggest?