But these startups are trying to take market from the incumbent Google, and increasingly Facbeook. These established businesses have paid back their investment and are now cash cows for their owners. The flows for Adsense for example will be in balance between cash in from advertisers, and cash out to publishers and Google shareholders. I've been in all three of those positions, often simultaneously, and been happy with the outcome.
When the VC tide goes out Adtech startups will be running up the beach trying to cover their white bits. But with less loss-making competition it could be very good for Adsense and Facebook.
As an aside, I see a future where digital advertising is equally split between Google for transactional (buy something today) adverts, and Facebook for brand awareness and engagement. I think Facebook today is in the same position that Google was when it was only selling ads against its search result pages. When Facebook launches adverts beyond its own property then the cash is really going to start flowing.