Followers on Substack only is much less valuable than a subscriber for which you have the email address.

This is a first step for Substack to control who see your posts, when, and insert ads amid your content. You also can’t export followers like you can with subscribers.

I also suspect that a free subscriber is more likely to become a paid subscriber than a follower.

So they diminish your earn potential, decrease the audience of your longer form writing (usually the reason someone creates a newsletter to begin with) and lock you in, making you lose control on how you communicate with your readers.