In theory, some of the ad networks offer targeted advertising. In practice, they seldom deliver on that promise. And even when they can deliver targeted advertising (such as airline ads on travel sites, for example), the targeting and the CPM isn't likely to match what a niche site on a commercial site can earn from AdSense ads.
I don't see why. Traditional mass advertisers don't use text ads.
The latter certainly won't happen. And Google can't tighten up its definition of what it considers to be "competing text ads" without losing the publishers it needs most. If it were to forbid affiliate links, for example, it would lose sites like mine that earn twice as much from affiliate links as they do from AdSense. The more commercial a topic is, the less willing a publisher will be to let Google dictate terms. The people at Google are smart enough to realize that demanding exclusivity would relegate AdSense to being a program for hobby sites, information sites on topics with little commercial potential, and other Web sites with limited revenue options.