> most tech authors including Douglas Crockford, John Resig, etc will not be able to pull $70k in 12 months whether they self-publish or let a traditional publisher like O'Reilly market the book.
I think they definitely could if they self-published, but not if they went through a traditional publisher.
The cut that a publisher takes is insane compared to the value they provide. For fiction, it may be worth it because they act as a quality control filter and send a signal to readers that the book is may be better than the sea of self-published novels out there.
For technical books, the market is totally different. It's not that crowded, and most readers will find the book through you, the author, and not through bookstores or the publisher's own marketing efforts. In fact, most technical publishers aren't even interested in you if you don't already have an audience.
A publisher might help you get on bookshelves (though you can do that while self-publishing too), but I haven't seen much evidence that they will otherwise significantly increase your sales. Given how big a slice of the pie they expect to take, you should be confident that they will do something like increase your sales by 8-9 times before they are earning their keep.
Publishers also help with the other production stuff: developmental editing, copy editing, design. I'm not convinced they are very helpful at the former—again you already need an audience and you are more clued in on what they audience wants than the publisher is. Copy editing you can just hire a freelancer to do and throw a relatively small fixed chunk of money at them for. Likewise for design.
If you decide to self-publish, you do have to do lots of grunt work yourself—managing mailing lists, dealing with freelancers, talking to foreign publishers for translations, etc. All of that is a drag, and there's value in outsourcing it.
The interesting question is whether you would be willing to pay a separate company something like 80% of your profits, forever in order for them to do that. Given that it amounts to writing a few emails and filling out some forms every now and then, that seems like a pretty steep price to me.