Another major shift that the article neglects to mention is global arbitrage. The advancement of camera + software quality and accessibility means people can make great pictures anywhere.
These days there are a lot of studios in the stock houses like Getty and Shutterstock that are based in Eastern Europe, or Asia, where the small amounts of money from stock images can really add up over time.
Copying styles, compositions and concepts of historically successful stock images means these producers make out quite well for their regional market by selling to more expensive markets.
Photo prices are going down for American photographers but it's still good money for smaller economies.