What if you have family or friends "familiar with your domain", but no paying customers?
Easier said than done. For most people that probably means updating literally hundreds of sites (and better hope all of those have a way to change your email). Not to mention telling your contacts to use the new email, and hope they don't forget and send an important email to the wrong domain.
Like I'm flabbergasted at people complaining about the burdens of doing something nobody would do, because the cost isn't worth the effort.
Now if they raised prices to $100+/yr I could imagine, but $15/yr is negligible costs.