You can simply put two buttons on the email, one for unsubscribe, one for re-subscribe. If they unsub by accident they can simply pull the last email and re-sub. It's not rocket science.
“Hey. You unsubscribed. Here’s a link to resubscribe if you happen to want to!”
Right after someone unsubs.
It’s already pretty standard practice to send an email notifying that the unsub request was processed.