If you look through your email inbox right now, I'm sure you'll find hundreds if not thousands of emails that have no use to you.
You subscribed to those lists at some point, out of curiosity or maybe by mistake… and now you're so indifferent to the emails you're getting that you don't even bother to unsubscribe.
You've become the thing you should fear when it comes to managing an email list.
Indifferent subscribers are like zombies, they just fill space and slowly decay your list.
Your open rates go down, your email deliverability becomes poor, and eventually your domain reputation will be so low all your emails will hit spam.
To avoid list decay, you need to keep your subscribers engaged. Give them enough variety so they don't become indifferent to your emails.