Why is the PPP still $400 and no stable OS?

[Kevin Kofler]

btw, do you remember freedomphone by eric finman ! almost like a scam product. ironically based on mediatek chips. mediatek's chip security was very questionable.

As I already pointed out, the BraX3 is also based on a Mediatek SoC (MediaTek Dimensity 6300). (Source: this Notebookcheck article.)

The description in the link says it can run Ubuntu Touch, so I am not sure what you are referring to with "probably not compatible with mainline-kernel-based GNU/Linux."

As I also already pointed out, Ubuntu Touch is not mainline-kernel-based. It uses Android kernels and Halium:

Why does Ubuntu Touch use the Android kernel?
We rely on the Android Linux kernel and drivers to make the phone work. This is the reality in the world of Android hardware.

The only exception being the "native ports", which, as you can see on https://devices.ubuntu-touch.io/, are only the 4 PINE64 device ports (and an old Raspberry Pi 3 port of the ancient Ubuntu Touch 16.04 that was never updated to 20.04).