A fixed rate ad model would be the death knell for google.
Some verticals are so expensive I'm surprised that the FTC hasn't poked their head in with a look towards predatory/discriminatory pricing laws. That being said I think the market will probably fix that issue before any regulatory body gets to it, google in particular has been very busy embedding itself into the power corridors via crony appointments to the appropriate oversight bodies. For the time being they have a pretty cozy relationship with the folks in charge.
Crappy search results, overpriced advertising model, the void will get filled soon enough, the space is actually over ripe for another disruption.