Most were born as natural born subjects of England.
“...few of our prominent citizens were natural born citizens...” True, but the Art. II language goes on “or a Citizen of the United States, at the time of the Adoption of this Constitution...” So the Founders were clear as crystal on this requirement, 14 years post founding. They anticipated that their generation would not be around for but a few more elections and were likely biased toward those who were born here and Citizens in 1789. So, for example, Alexander Hamilton was unlikely to be considered as eligible.