PostmarketOS reminds me a lot about the approach Openmoko (https://en.wikipedia.org/wiki/Openmoko) took way back over a decade ago, except that they did have to build the hardware as well.
There've been several such prominent open source handheld projects in the last 20 years. PostmarketOS seems the most viable, IMHO, because it emphasizes the old Linux (and GNU) principles of getting working on lots of available hardware, with true open source, mainlined. (Separately, it also potentially builds on a wealth of GNU/Linux stuff that there's no good reason shouldn't have a lot of commonality between desktop and handheld.)
