Does deleting inactive email subscribers really boost your delivery and sales?

Our email list hit 40k subscribers, but open rates dropped below 12%. I was terrified to clean the list thinking a big subscriber count meant more potential buyers.

We finally deleted 12k people who hadn't opened a single email in six months. right after open rates jumped to 28% and weekly email revenue went up by 15% because main inbox placement improved.

How often do you prune inactive contacts from your newsletter list?