I use LineageOS right now. It's great, and I'm glad it exists, but it still requires me to write the majority of my apps in Java and to test them in an emulator, and it still requires me to use the Android OS for everything.
I know I can theoretically compile my own ROM or something to customize the desktop environment, but... bleh. It's time-consuming for me to build things on Linux, but it is more time consuming for me to build things on Android. Android development is just a pain all around.
This on the other hand, isn't just a >$600 phone. It's a >$600 grimoire[0]. Expensive, and cumbersome, but nearly all magic is. There's an almost subconscious brand (for lack of a better word) that devices of this nature fit into. They're trading user-experience for dev-experience, which is not something LineageOS or Android is optimized for.
As to the rest of your criticism, all of your other points seem completely fair. I'm not sure what I would put the odds at of the device succeeding (note, succeeding means finding a stable niche, not beating any other device). And while I can forgive a lot of flaws in software, for the price they're asking the hardware/drivers need to work well.
I'm very much in the "wait and see" camp. But I do see a lot of potential.
[0]: https://news.ycombinator.com/item?id=19253038