When AdSense was introduced, pricing was artificially high, because advertisers had to opt out of the content network. Advertisers quickly got savvy, and less than a year later, Google introduced "smart pricing" to create a better alignment of price and value. More recently, Google introduced separate bidding for the search and content networks. All of these changes were good for advertisers, and they've obviously worked well for Google, too, because AdSense revenues continue to grow every quarter.
What hasn't been good for existing publishers is the sheer growth in the content network and ad units. As another member once asked, "How many publishers haven't added pages [or ad units, in many cases]?" If the total numbers of publishers, pages, and ad units are growing faster than advertisers' expenditures, the pie is going to get sliced into smaller pieces.
On the bright side, advertiser tools such as placement reports and unlimited domain filtering are welcome innovations for publishers whose traffic converts for advertisers.