Nielsens are only one factor in TV right now. HBO and Showtime don't care about Nielsens because they have no advertisers to please. They do
care about ratings as an indicator that a show can attract and retain subscribers and they care a lot more about a show's "buzz" (among critics and the general public) than broadcast networks do.
The changes to TV will be this:
A lot of what we know as "TV" won't change at all, or will change incrementally.
The new kids (Google, YouTube, Yahoo, Netflix, et al) will be different from traditional TV in being (generally) cheaper, nichier, far more abundant, and with more of an interactive element, ranging from minor (comments) to major (scripted dramas that merge with games at the extreme end of the spectrum.)
This will result in a huge range in quality, subject matter and form to sort through, so success depends heavily on people having access to powerful search tools.