Did you read the article or just the word "React"? They're incrementally migrating, not fully rewriting the application.

Yes I did, and did not saw the business case (monetary) in the decision.

Well the article is primarily about the technical implementation, not the business rationale. But they still mention the main reasons: engineer productivity, community support, and performance gains. All of those translate to money.

As someone who used to work on large Backbone apps I agree that it's a huge drain on developer productivity -- hard to add features, hard to test, lack of standards, etc. Not to mention the recruiting problem: I would never accept a job offer to work on a Backbone app again.