This new speaker from Amazon could easily be the result of a partnership between Amazon and Sonos. That would answer the sound quality (Sonos) question and the smarts of the assistant (Alexa) question in one fell swoop. Sonos with Alexa integration has already been in private beta for some time. Slotted between the Play 3 and Play 5 @ $300 - $350 you have an immediate direct competitor on sound quality, smart assistant, and price. Win/Win for Sonos and Amazon. No longer is the decision do I want a less intelligent smart speaker with good sound or a more intelligent smart speaker with lesser sound. An Amazon/Sonos amalgam offers both. Pure speculation by me, but not unreasonable speculation.
Everything you mention totally makes sense. Competition is always good. People will just need to choose whether they value privacy or not. Amazon makes money by selling you products, I don't feel comfortable putting that in my home. Apple Music will also play a factor as you'll only be able to get that integration from the HomePod. It's good to have choices, I don't think it'll eat into Apple much though.