Youtube does not Purge Dead subscribers, otherwise inactive ones. It does however sometimes wipe away accounts that have either been deleted or removed for one reason or another. So, in turn, you will "lose" a subscriber when honestly you already did. However, Youtube doesn't get rid of people who just don't log in anymore, or don't watch your content anymore. They will just sit there. I mean Pews has millions of "dead" subscribers, the guy gets average views of just a few million compared to his overall number. It is just a thing that's a part of youtube