Yeah, I'm doubtful whether I'd buy one myself indeed. That doesn't mean it's not for everyone though, and since I don't know your exact preferred use case I thought I'd at least mention it, since it is an interesting device none the less

Regarding your last point, that Linux is a promise without proof, that's not entirely accurate I think. Planet Computers' previous device, the Cosmo Communicator, is actually running Linux in the form of Debian with KDE Plasma DE. It runs on a modified kernel with closed source (Android) drivers though, so for me that's not good enough. I can imagine for most people that wouldn't be problem at all though. I think it's safe to assume the Astro will run Linux in a similar form since it's also driven by a Mediatek SoC.

If open source/mainline kernel is important to you, did you consider the Purism Librem 5 or the Pine64 PinePhone?