These numbers are based on W3school users. People that go to w3schools.com are interested in web development. I am guessing the average IE6 user is not interested in web development, so I'd be wary of the numbers.
Test in IE 7+. Tack on http://ie6update.com/ Call it a day. We've done this with clients in healthcare, construction, accounting, etc. and all have been pleased.
I'm more concerned about the percentage of XP users. IE 7,8,9 on XP do not support things like SNI because they are stuck on an older network stack in the OS. Microsoft needs to force the end of life for XP.
I've recently dropped it; I simply say to clients that if they want their site to support a ten+ year old browser with little standards support then costs will increase. Haven't had a taker so far.
Those numbers don't represent browser market share.
I'm kind of surprised at the free-fall once it got below 5%. At this rate, in 3 months less than 1% of traffic will be IE6, so it should be finally safe to drop.