Avira, we had haskalah and Reform a little earlier than 1920. Maybe this is when they arrived to Brisk and even that is not so. There were probably always people not interested in observance, but they often did not have any good options, so they stayed quiet, and maybe their children or grand-children straighten out .. In cases when they could – Greeks, Spain, Germany, they left.
And I don’t think majority of “klal” has a lot of urgency about the issue. How many BTs are in regular shuls? how many ever meet or talk to secular reform Jews, Israelis or Russians? How often do Rabbis talk about outside other than about the danger? In most places, not often.