I believe ad networks (that rely on third-party scripts and cookies) days are numbered thanks to adblocking. The only ads I see surviving long term are affiliate links, sponcered content and static native ads.
The problems with ad networks are too big to go on for very long. Poor user experience, Ad fraud, malware, the indiscriminate gathering and selling of user information. It's just a matter of time 'till everyone has a adblocker.