Back when I ran email, we consistently saw that about 8% of purchasers - who we regularly sent email to - never opened emails we sent to them. Several industry pundits recommended that I stop mailing these people as they were "not engaged."
Just because people don't do what you want when you want, it doesn't mean they're not engaged. It just means that they're engaged in a way that either you don't expect or you can't measure.
For example, some people just don't trust the email channel. Maybe it's one too many attempts at transferring millions from secret overseas accounts but there are some folks who just will not execute via your email. Even if it were the best email in the whole wide world. They simply won't. What they will often do is either go directly to your site (usually prompted by your subject line) or search based their idea of what your email is trying to get across.
So do you abandon those folks in the name of efficiency? You could...but it turns out the email still has value - just not the value you think. Will they purchase even if you don't send them email? Why take the risk? So the real issue here isn't the email, it's the fact that your measurement systems are myopic. Or that you're stuck with a punitive and counter-productive CPM agreement with your ESP.
I'm really beginning to become wary of "email experts" who are anything other than deliverability specialists. Email is simply an arrow in the marketing quiver. In some quivers it's a huge arrow. In some, it's nothing more than a sharp toothpick. Before you go changing arrows, make sure your measurement systems are up to par.