It is extremely rare that the cost to support a browser outweighs the economic importance of those using those browsers. If it is then you are probably too small an entity to matter anyway, but as soon as your IE6 users are numerous enough that you can assign a percentage to the population you would probably be well advised to simply serve them rather than to attempt to force them to upgrade, if they haven't done so yet they either have very good reasons or you won't be able to convince them anyway.

Every browser has security issues. And as a business it is not up to you to decide what browsers people use, the best you could do is inform them but the decision is up to them.