The 14th amendment was ratified in 1868. The total population of the US in 1870 was around 38 million. The total number of immigrants was around 2 million. The slave population was around 5 million.
The implications of the 14th amendment are clear today and they may even have been clear then. Nevertheless the most pressing, most politically salient issue was the newly freed slaves. In one day (the day after ratification) the United stares gained 5 million new citizens.
I would not be surprised at all if the people involved at what time were a tad bit myopic, with respect to the problem at hand.
EDIT: In fact in 1860, the us census measured 4.1 million people not born in the United States. Is your claim honestly that while the census actively measured this, not a single person realized what birthright citizenship would mean for the children of those 4.1 million non-natives?
I really hate that usage of the word.
I mean, your basic argument is that Thomas is just pointlessly making semantic disagreements with the majority that would have no effect on this or any other conceivable case because either basis for incorporation produces exactly the same result not only on the present case but all possible cases. While I have a low opinion of Thomas, it's not quite that low.