Expecting people to speak their commands in public just seems intrusive and I don't understand companies trying to pretend it's a norm to sell their products. I really don't want to know what song the person next to me is listening to. That's the point of headphones.
People with visual disabilities often already have better tools to interact with their phones with privacy, and nobody minds people using voice for people with physical disabilities either (and these tools exist already too).
Related: it's about to be even more weird for everyone in the world named "Alexa".