300 years ago everyone was either Chareidi or humongously off the derekh (meaning becoming a member of the local state religion be it Christian or Islam, or joining a “reformist” group such as the Frankist or the followers of Shabbatai Zevi). Those who were willing be mesiras nefesh for Torah and Mitsvos converted (which offered immediate economic benefits). There were no “modern Orthodox”.
Remember that until the mid-20th century it was perfectly legal, and in fact was public policy, to discriminate against those who didn’t do their jobs on Shabbos. Just being Shomer Shabbos represented tremendous mesiras nefesh (and in that era, “modern” Orthodox synagogue often had people who drove to shul and parked a block away, and members who felt proud that they kept kosher at home while avoiding pork when eating away from home). In that era, being Shomer Shabbos and going to college or holding a job for a non-Jewish employer (including the government) was difficult, so anyone even a bit frum was regarded as a fanatic.