No disrespect for your valuable discovery but this attitude of “it’s possible, if you do these non-obvious steps” feels a lot like victim blaming in UI.
If Apple (or anyone else) wanted to make a feature used, they can. For everyone else, if Siri is off CarPlay doesn’t work. And that’s by design.
Not the design of “ooh if Siri is off then voice in CarPlay won’t work” (warnable), but punishment if Siri is off.
Again this pattern isn’t Apple only but it’s bad everywhere.
It feels cynical to see this as a punishment when it's such a specific use case that does demonstrate deep integration with Siri. Maps, Messages, etc. use Siri for their interactions.
I am sure that there was a meeting where they decided what to do when Siri was off and somebody decided (very possibly with ulterior motives) not to split the feature set - all or nothing. However I don't think the challenge they were faced with in this hypothetical meeting was an easy one.
The alternative is you open the Messages app and you can't send messages. You open Maps and you can't get directions (unless parked). Sure, I get that they could show a screen saying "Sending messages is not available when Siri is disabled" but now you're hitting error messages while driving.
Anyways, the main reason people would disable Siri is accidental activation, and Apple provides all the toggles needed to avoid that without disabling the core components needed for CarPlay.
Before you start driving, at stop lights, while waiting in lines, etc.
I don't know how it works on CarPlay but when I turn my car on I have a bunch of suggested addresses (home, work, parents, recent Maps searches, etc) that I just touch-to-go. Having to use voice every time you want to navigate not only sounds unnecessary, but cumbersome.
I purchased an iPhone 15 a few months ago and ended up making this discovery myself. CarPlay would refuse to launch unless I enabled Siri. I didn't do any of the Siri setup, or anything but the app would hard refuse to launch unless I went and toggled on Siri. Maybe that's different depending on your make/model, or the specific infotainment system in your car, but in my '21 Kia Forte, Siri is a very hard requirement.
You don’t need to enable Gemini or voice assistant on android to use android auto. Some functionality is lost, of course, but you can still navigate and play music.