They’re a fabulous Bluetooth speaker if nothing else

HomePods "synergize" well with Apple Music subscriptions.

I also think they're a protective measure to keep people from getting someone else's smart device, which might lead you into someone else's ecosystem.

I think this is a fair point. Taking a look at the product page for the $300 HomePod (https://www.apple.com/homepod-2nd-generation/), I would argue that the HomePod is marketed more as a speaker with Siri integration rather than a Siri device with a really good speaker.

The bullet points I see on the above page, in order, are - really good speaker - Siri integration - integrates with Apple ecosystem for playing music - smart home integration - "private and secure" whatever that means

So, it's not clear to me that "a better Siri" would necessarily result in selling X amount more speakers to make Siri a worthy investment while investing in better audio technology does because it's the main selling point for HomePods.

Homepods are great to pair with Apple TVs for stereo sound.