Everything you say is correct, but the one thing I actually miss on our Tesla is Apple Maps. I have liked it better than Google Maps (or its Tesla version) for a long time -- like its turn/direction instructions (telling you the turn is at the next traffic light or stop light or ... is a lot better than the distance you get from Google Maps).
And, ironically, I was eager to try CarPlay on a recent trip where there was no Tesla rental available, we rented a VW Tiguan, only to discover that for CarPlay to work you needed a USB-A to Airplay cable and I only had USB-C to Airplay cables... I even got a third party USB-A cable at a gas station, only to discover it too didn't work (likely it was cheap POS where they only ran the wires needed for charging). So thanks to VW not supporting USB-C (which our 2020 Tesla Model Y certainly does) I never did experience CarPlay...
Apple loves to canibalize itself. iPod, which used to be 50% of the company: practically gone, totally canibalized by the iPhone. The iPad has already eaten plenty of Mac, outselling it between 2:1 & 3:1. The idea that if only the iPad were less locked down it would sell more and canibalize the Mac, thus Apple doesn't allow it, is absurd.
The App Store "tax"? Sure, Apple doesn't mind the cash. But they are first, second and third a hardware company: that's where the real money is. The reason they have no intention to allow side loading apps on iOS has to do with user experience, eliminating support headaches and security (order may be different, but these re the reasons).
The fact is your dream device would appeal to the same people who buy desktop Linux machines now. They exist, but they are a tiny part of the market. Nobody can stay in business catering to just those customers.
And, ironically, I was eager to try CarPlay on a recent trip where there was no Tesla rental available, we rented a VW Tiguan, only to discover that for CarPlay to work you needed a USB-A to Airplay cable and I only had USB-C to Airplay cables... I even got a third party USB-A cable at a gas station, only to discover it too didn't work (likely it was cheap POS where they only ran the wires needed for charging). So thanks to VW not supporting USB-C (which our 2020 Tesla Model Y certainly does) I never did experience CarPlay...