Killing off products that don't have enough interest doesn't seem to be stupid at all. That's how they shift dev effort onto developing new services and extending services that do have sufficient interest.
It remains a technological product, though, and one doesn't like to buy into tech ecosystems without a full assurance of ongoing support. Chromecast Audio being discontinued today is one thing, and Google does promise continued support for the product … but Google's track record may not bear that out.
In the case of some sort of critical security update or a theoretical future update to pick up a hitherto undetected, show-stopping bug, there's little assurance Google would look after its customers — they've too much of a tendency to introduce something then rescind the product not long later, something that doesn't inspire much confidence in whether or not there will be ongoing, high-quality software updates to support the products.
