Yes, NASI is the motzi in terms of action, I meant that the migration point is the motzi in terms of the theory.
You’re right about otd/bt. What I should have said is that the difference doesn’t seem to be nearly enough to fully explain the numbers, and in no way negates the demographic reality of age gap/population growth causes gender disparity, and that closing the age gap still helps solve the additional disparity that migration might cause.
I agree that for the most part, twenty year old litvish bochrim shouldn’t be getting married. But there’s not nearly enough downside in focusing shidduch efforts on the twenty-one plus girls to offset the tremendous benefit.