Thats true, however you would still require a powered speaker, or an amp and a method of getting power to it. For bathroom installations for example, I have a sonos connect amp and two ceiling speakers. The amps for each bathroom are located in rack in another room. They just have speaker cable to the ceiling speakers from the connect amp.
If you move to chromecast audio, then you will need to feed that audio from chromecast into some form of amp, which then powers the speakers. This of course could be done with a cheap stereo amp, and again rack mounted somewhere unless you want an amp in the bathroom. Not as neat as the sonos installation, but workable.
If you wanted to replace the single countertop play 1 in a kitchen for example, having a chromecast audio plugged into a powered amp and then a speaker would get messy. Having it sat on top of a simple powered speaker may be better, but still not as neat.
For casting audio to a single stereo hifi, then yes chromecast would be fine. For true multiroom audio and neat installations, sonos still takes some beating imho. Its also infinitely more wife friendly and acceptable to she who must be obeyed. All imho off course.
Right now I can walk into my house and do the following:
say “Alexa, ask sonos to play Elvis Presley in the Kitchen”.
If I want to I can say “Alexa, ask sonos to join Lounge to the Kitchen” - it now plays in both rooms. Or “Alexa, ask sonos to change room to Lounge”, and it starts playing in the lounge and stops in the kitchen.
Will Google home have that ability with chromecast audio - almost certainly. But it will still require some thought put into getting powered speakers and connecting them to a chromecast audio unit. It will also require decent wifi coverage as they dont create their own mesh network separate from the home wifi.