People are going to switch away from Google search, but Google's ad networks (DoubleClick, GDN/AdSense, Admob, AdX) actually compete with other ad networks (like OpenX, Rubicon Project, Criteo, AppNexus).
Many publishers use more than one ad network to maximize their income, so advertisers can just move to the other networks.
AppNexus is now AT&T Advertising and Analytics as of today..
But only Google ad networks appear on Google products. They can choose what advertising appears there with no competitive impact.