The stats I’ve seen have shown that immigrants from Schengen contribute more than average and commit less crime than locals.
As such on a purely data driven view there is nothing wrong with Schengen. For most people the issues isn’t with movement within the people that have lived in Europe for generations. You know they are European, they have European values and there is a back and forth movement/ different perks. In a way Schengen works the way it was designed and works well.