There is no such thing as 'the ultimate headphone' that you can buy and never be tempted by anything else. There's only the best available at that time. At some point, the HD800s will be the budget model and sell for $99. But there will still be a model that costs upwards of $500 that every reviewer creams their jeans over. Personally, I still love the HD600s - I can't find anything to dislike about them. If I upgrade the rest of my audio gear, the HD600 will sound better again for quite some time. I'd need to spend thousands of dollars on other stuff before they became any kind of bottleneck. It makes no sense to pair a top of the line SLI graphics card with a crappy CPU and 1gb of sluggish RAM. By the time you upgrade the rest of your shit, the card will be out of date again. DC had a great amp already so pretty much any headphones he bought for less than what he paid for the HD650s would have done it a disservice. Not everyone is in that situation.
Also worth bearing in mind: these things hold their value pretty well because audiophiles tweak and 'upgrade' constantly (inverted commas used because most of the time, the upgrades are all in their mind...). People will buy the HD800s, love them for a week, then turn on them with furious anger and sell them on eBay for $1,375. So if you feel you want to upgrade later on, it's going to be a lot easier than with PC gear.