You’re partly right. If anyone signs up at all, then it’s a net gain in ‘conversions’. But it’s not a majority at scale, and it doesn’t have to be anywhere close in order to work. Typically only a small minority of visitors convert to being email subscribers, and then only a small minority of those email subscribers convert into paying customers. It’s not uncommon for the paying customers at the bottom of the funnel to be in the single-digit percent of visitors, or less, and the people annoyed by popups who leave to be the overwhelming majority.