Interesting. I'm guessing it has to do with the market and perception, not a DMR issue. Expecting people to use a Chromecast Audio dongle to stream to audio equipment is a big leap for most people. It is was it is unfortunately.
Chromecast audio will still work with Google Assistants and with the Google Home Max (smart speaker), also any other product with it build in. They also say you'll still be able to stream audio to Chromecast Ultra just as you did with CA. Too bad they were sold out of the CA before I got to grab another one! Though, I have 4 total; 2 dongles, two built in.
I'm kind of not liking the current trend. Too many ecosystems that are one to themselves. They just don't play well with others; musicast, chromecast, sonos, hios. homepod, and whichever ones I'm missing.... But at this point in time, that's what's selling. For instance, people are happy buying an XXX Assistant for every room to play music and ask dumb questions. They're happy with the quality too:-(
CA was very cheap and kept me out of buying into this expensive dedicated mess (in the sense I would not be buying system I REALLY want) just to sync music through my house.