Just because a subscriber doesn't open an email -- it doesn't mean you should stop emailing them. Turns out that tiny messaging in a consumer's inbox can influence purchases. Email campaigns prompt Brand Activity (direct entries and brand searches) from folks who prefer not to click. This can be measured.
MailChimp conducted an analysis of 6.6 billion email sends and discovered that "inactive subscribers" had a higher order frequency and average order value than "active subscribers". Inactive subscribers were defined as those who did not open or click on an email over the past 6 months.
No doubt, "scrubbing" lists goes a long way to explaining why the retention rate is lower than that of "active subscribers". Rather than scrubbing lists of inactive subscribers, however, a better approach might be to invite this section of your list to unsubscribe.