Always illegitimate?
Saying that it is illegitimate after Congress would vote to declare birthright citizenship going forward is one thing,
That is one thing.
What you are saying is quite another...
British Common Law way before Lynch that undergirded U.S. law and set the foundation for it and the Lynch case and others both before and after that (1985) work in harmony with the 14th amendment, but not opposed to it.
This is a topic I have researched quite extensively, and there is no consensus in 1787 that the US followed British Common Law as regards to citizenship. There is much evidence that we did not do this.
Most of evidence on the Pro-British-Common-Law side comes from William Rawle, (In the guise of his very popular book "A view of the Constitution" which was widely distributed and regarded at the time as a pivotal work on the Constitution.) and I have uncovered a great deal of information that leads me to believe he was deliberately lying about the US following the British Common Law jus soli principle. I go beyond saying he was mistaken, he was willfully and deliberately lying about this because he knew better, and chose to deceive people anyway.
This is actually a very interesting subject, and I have not told a whole lot of people about what I have found, but those whom I have informed think I have a pretty good case regarding this.
Believe it or not, the effort to get people to believe we followed British Common Law and the effort to pass the 14th amendment have far more in common then you would guess.