As far as web accessibility and interaction-heavy sites are concerned, he is asking the wrong question. What he should be wondering is,
When is the right time for the fancy stuff?
Derek is of course aware of the importance of accessibility. However his approach seems to be to build the stairs first and then put in the wheelchair ramp later, and no matter how well you plan you will always come across implementation problems you had not considered.
My main criticism of Derek's article is that he constructs a dichotomy between business imperatives (getting the site up) and doing the right thing (implementing accessibility). However there are tangible benefits for your project in getting accessibility right early on.
Most importantly, there is one blind and deaf user that every web developer should be concerned about: Googlebot. If a site is not accessible for a human with a disability then it almost certainly will not be indexed properly by Google. You cannot improve your site through user feedback if you have no users because no one can find your site.
Of course, a big motivation web developers to make their sites accessible is that it's the right thing to do. And it is. But if you follow progressive enhancement and make accessibility part of your development process then you'll get more out of it than just a warm fuzzy feeling.