I think most people excited to try out pairing two of these things together aren't looking to replace an existing soundbar setup, but rather have an additional setup in a room that otherwise would just be making do with the TV speakers due to size.
For example, I have an excellent stereo in my living room. It gets much louder than even two HomePods would be capable of, and is pretty ideal for movies and TV shows.
In my bedroom however, I don't need or want a stereo that large. I do have a single HomePod in there though. It works great with my Apple TV. But if I could pick up a 2nd one (that's primary function is for another room) and occasionally pair it with my main one for a decent home theater experience in my bedroom, all the better.
This isn't a zero sum game. People who have interest in two HomePods could have a variety of uses for them. Home theater use is just one of many use cases.