This is becoming quite a tangent, but how heavily do you pressurise customers to upgrade? Do you just keep reminding them? Or are you more active than that?
The pressure that I apply is internal to my company, to keep us on current releases, to examine and test new features. I don't "pressure" customers on version upgrades, though I do strongly recommend upgrades or service packs if they're vulnerable to a serious OE bug.
For my customers the pressures are two-fold. First, as our development moves forward and we take dependencies on new OE features, that means our minimum OE release for the current version of our software moves up. So when the client has a business need to upgrade to a newer version of our software, that almost invariably requires an OE upgrade as well. Second, in my industry (financial services), the clients have regulatory pressures, both keeping on a current, supported and patched OS (which means recent OE, certified on that OS) and staying on supported releases of third-party software, which applies to both our software and the products we bundle like OE.