Google recently announced that Chrome will start blocking 3rd-party cookies within 2 years. Some other browsers (I think Safari and Firefox) are starting to block them by default, therefore in my opinion they could stop being viable quite soon.
What do you think will happen to web advertising when when / if this happens? Will more advertising spend end-up in the walled gardens where tracking will still be possible? In turn will that have a negative impact on the open web or lead to the search for alternative funding methods?