IE6 users will only be motivated to upgrade when a significant part of the web no longer supports IE6. Unfortunately, most of the web will only consider dropping support for IE6 when its market share is considerably lower.
I wanted to go further, but then I realised that they don't even have a clear concept of what is the difference between Internet, Google and Web browser...and you want them to upgrade browser? ;) I think out of those 25% IE6 users, there is a large portion of users like this.