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.
