My small city in Washington is the template for the state's ADU law and multi-family density law. When the ADU law passed, we saw a lot of people convert garages into ADUs and anymore any custom builds almost always add an ADU. But to your point - it isn't clear how many have become rentals. What I observe is that many people saw it as good resale value, as multi-generational living, or simply a place to put guests when they came to visit. It was about the same time we put a moratorium on VRBOs, which had been another reason people saw these as beneficial without becoming a full-time landlord. Personally I have one and it has been beneficial income while also giving my tenants a good transition point (college students, people escaping bad marriages, people who need a place to land while they look for a place to buy, etc)