In some ways it's very frustrating, but I've invested heavily into the Apple infrastructure for my home cinema and HiFi. Networking, HomeKit, AppleTV, airplay, etc, but now I've lost the choice to use my own speakers which I've invested heavily in.... here's hoping for a "dot"!
According to Apple, the HomePod will control Airplay 2 devices:
"You can also control any other AirPlay 2‑compatible speakers"
This means I can use it with my main system that has an AppleTV 4k and my other systems that have Apple TV 4s. That will allow me to use the HomePod to control what is playing on any of my three main speaker setups. I am hoping that my Denon and Onkyo receivers eventually get updated for Airplay 2 for more convenience (since they already support Airplay). However, the AppleTV is a great workaround.
Siri vs Alexa - Alexa doesn't work with Apple Music. That is its main downfall for the 30 million plus Apple Music subscribers. For me, that is a big deal. The HomeKit functions that I use work as well with Siri or better. For example, Alexa doesn't know what it means to set a Hue light "white" without some third party skill. Siri does. Since it is very common to want to turn a Hue bulb back to white, it is an inconvenience....unless you want to also memorize a command from a third party.
I have Echos and love the Dots because of the aux and bluetooth (in and out). However, for my main systems, I prefer Airplay because it sounds better in my setups. That is probably because of my receiver's DAC.
I will most likely keep my Echo Dots because of their different third party skills, but I can see a HomePod being very nice to have for when I don't have time to sit down and listen to my main system, while still getting great sound quality. When I do have time to sit down, I prefer Airplay/AppleTV digital input into my receiver versus the analog out of the Dot and as I said, the HomePod should control them by voice.
I should mention that a Google Home Max + Chromecast/Chromecast Audio does something very similar to what I am describing above. However, it also lacks Apple Music.