This may be true for TED (never been) but for most tech conferences I've spoken at the presenters are still putting together slides for their presentation right up until they talk. This tends to get the latest stuff in. Not to say I'm disagreeing with you -- quite the opposite, in fact. With rare exception, during most of the conferences I've been to the "hallway track" has been more valuable/informative.
Given that the talks are canned, and the materials they are based on are almost always published, why should I pay for a conference when I can already leverage existing networking/social channels to stay in touch with the latest?
It's hard to go out for a beer with someone who isn't in the same place that you are. And people will definitely tell you stuff over a beer or two that they won't write about in their blog.
You don't need a conference to go out for a beer, and you certainly don't need an exclusive conference to go out for a beer with someone interesting.
You do need a way to gather interesting people in one place, but you don't need a traditional conference (especially an expensive conference) to do so.
50% of the value is the content of the talks, which supposedly described the latest and greatest.
Just like in academia, the talks are actually dated: they have been prepared months before and don't really represent the latest. To know the latest, you need to chat with various people to find out what they are working on right now, that they can't quite announce yet.
What's on stage is canned, in a sense. If you want to be in, you got to be there in person to catch where the future is.
This has held true for me both in academia (research) and web 2.0.