If you’re an Internet company trying to pull advertising dollars away from TV, one of your arguments is that you can do much better tracking. When you run a TV ad, everyone watching the show gets the same ad, and people watching different shows see the same ad a non-optimal number of times.

That's a very good point. This would definitely happen to some degree. But it would still hurt internet advertising a good deal not to be able to track uniques, to show you an ad only once per day, to count how many people have been shown an ad, etc. even if you could know about how many people were on the site.

The result would likely be tracking via login as I describe, rather than true anonymity.