I think high CPM rates will also always exist, just on a much smaller scale. Right now, the big spenders on CPM ads are doing it as a comparison of a cheaper outlet to TV or print ads. They've got these massive unlimited budgets and the ad networks and publishers make a killing off of these idiots because they figure, if enough people see it, then for the sake of the brand awareness, we've accomplished our mission. Little do they know is that they can be getting sales, clicks, and leads for about 3/4 of the price they are spending on CPM's.
Will ad networks tell them about it? Gradually. I think one or two big ad networks will slowly go into a small war with one another and seek out the bigger paying clients by showing them better conversions. They'll probably mix 10% of PPC, PPL and CPA in with a 90% CPM campaign, showing the client a magic higher conversion rate on their campaign, and taking them on for even more money. This will go on for a few years until finally one day, in a very very long time from now (in real years, not internet years) CPM may become a thing of the past, although I doubt it.
Movie studios want brand awareness on new films. Because you're not going to buy or reserve a ticket from a flashy ad that annoys you on a website. They are willing to shell out some major dollars to make sure you and your friends see it for the 2-4 weeks of promotions. After that, it's on to another movie, and the same deal goes with that. These guys have $50M+ budgets. CPM is going nowhere for now.