People buy full-sized desktop PCs today either because they want the best performance possible or they want to save money by getting the best performance per dollar. Purism can’t compete in either the performance or economy market. People who want the best performance today are going to buy a Ryzen CPU, but Coreboot has only been ported to Ryzen APUs for Chromebooks and there is no way to neutralize AMD’s PSP. They are going to want an nVidia GPU which requires a proprietary driver.
As far as I know, the last desktop PC that got a Coreboot port and supported discrete graphics dates from 2014. Right now there are Coreboot ports for 10th gen Comet Lake-U and Ryzen APU boards used in laptops, but I don’t see any for Comet Lake-S or Ryzen 3000 boards used in high-performance desktop PCs. Google is able to do Coreboot ports for mobile processors, because Intel, AMD and Qualcomm want a piece of the Chromebook market, so they will respond to the questions from Google engineers. If Purism calls up Intel or AMD and starts asking detailed questions in order to do a Coreboot port for a desktop processor, its questions will probably go unanswered, because Intel and AMD aren’t going to waste time with a company that sells a couple thousand desktop PCs per year.
Even if Purism were able to convince Intel or AMD to answer its questions to do a Coreboot port for a desktop CPU, there is no business case for doing the work. It is much cheaper to reuse Google’s Coreboot work on a mobile CPU, than to do new Coreboot work a desktop CPU. By the time Purism finishes the work, the desktop CPU will probably be a generation or two behind the leading-edge tech, so there will be limited demand for Purism’s desktop PC.
Of course, Purism could offer a desktop PC without a Coreboot port, but then people would ask why not build their own desktop PC and install Linux on their own, which would cost much less than what Purism charges. Purism needs large margins and it simply can’t compete in the economy market.