Here's the core problem as I see it: the old (advertising) math no longer works, and that's getting people into trouble. For traditional advertising in print, radio, or television the math is simple and it's been refined over decades. Generally speaking you take the number of eyeballs or ears, you multiply by a certain factor which takes into account things like demographics and whatnot and then you're done. In large part it doesn't matter whether someone is watching murder she wrote or magnum PI or reading the paper, for each format there's a degree of effectiveness of advertising and a degree of lucrativeness for the target demographic. There are of course complications, but in broad strokes this is the way things worked.
Those assumptions completely fall apart on the web. Because the web is not just passive. It's interactive. And that changes the equation tremendously because it means that the effectiveness of ads can vary by an enormous degree depending on activity. When people are searching, for example, relevant ads (not just random eyeball hijacking brand promotions) are actually useful, and thus highly effective. When people are trying to communicate with their friends there probably isn't a good advertising context at all most of the time and almost all ads will be seen as useless interruptions and thus vastly less effective (they won't be relevant and can't as easily be made relevant). And there is a huge variety of other activities on the web that don't fit into convenient molds.
Now, does that necessarily mean that advertising revenue is a thing of the past? Obviously not, as in some cases there are circumstances where the potential for effective advertising is even greater than with traditional media. But it does mean that the math is far more complex. And it might mean that just because you have a direct line into a huge amount of eyeballs you may not be able to monetize that traffic to the same degree (by orders of magnitude) as an equivalent amount of traffic of people engaged in a different activity.
Additionally, we've seen the writing on the wall in terms of "eyeball hijacking" highly interruptive advertising in the modern age, it's just out of date and out of touch. Monetizing web traffic effectively requires a much smarter approach. In the ideal case advertising is so well targeted and so effective that it is welcomed and often not even seen as an ad.