Bing also gives Microsoft: - a relational knowledge graph - a way to annoy Google
"Annoy Google" is a "commoditize the complement" play.
Bing (and the Bing powered Yahoo + DDG) create a viable alternative where otherwise there is none. Google now has to constantly spend to keep their search tech ahead, and Mozilla and Apple can now threaten to switch default search engines forcing Google to pay protection money. Altogether, Bing forces Google to burn around 15-20 billion per year. This is money Google can't use to poach Microsoft engineers, or push G-Suite against Office, or Chromebooks against Windows, or Google Cloud against Azure.
Microsoft already has open sourced many key Bing technologies, and actively pays consumers to search with Bing. The primary goal isn't revenue, it's commoditizing search.