Care to explain why not? What stops someone who mined 10k bitcoins in 2007/8 from cashing out. If I remember correctly, there was a guy recently on #startups freenode who showed everyone a screenshot of his wallet at $800k[1] (one sec let me find it in my history) and claimed he mined them all really early.
They are definitely a handful of people who have over 10k bitcoins, but they're not storing it on an online blockchain.info wallet (which is where that screenshot is from). They're also not showing it on IRC.
That kind of screenshot can easily be produced by changing a little HTML on a 0 balance account.
