That may be so but there are plenty of people without an Apple Music subscription. There are also plenty of people who want to use their local music library. And, as mentioned, I want to use the HomePod mini as a chime as well (without a subscription).
I stream music from my local music library using Logitiech Music Server in a Squeezebox ecosystem. The HomePod Mini is acting as a Squeezebox device to LMS, using AirPlay. This works great.
And if that's the way Apple wants to go with the HomePod then they need to clearly advertise it as such. The HomePod Mini might have a "low" purchase price of $99 but if you need to spend another $10 each month just to "get the most out of it" then it becomes a very expensive device.