Ok, the whole premise of any advertising on your site that you aren't running yourself is that you aren't making as much money as you could be. Think about it, who is going to advertise for something that they can't sell for more than they are spending on advertising? The whole point of advertising is to spend less than you are making back.
The death of AdSense comes when advertisers stop converting and can't cover the spread. As long as advertisers get a good bang fo their buck, they will keep spending. The problem is that most sites where advertisers are spending 50 cents a click don't convert for crap and advertisers are wasting their money. To compound the AdSense problem, Yahoo and MSN offer better alternatives these days for PPC than Google's content network does.
That all adds up to less per click for AdSense perhaps, but the problem is a lot bigger than just Google allowing advertisers to opt out of the content network.