Great take, but if the market is expanding for capable PCs why are motherboard sales decreasing?

According to the article, because components are really expensive right now, particularly RAM and storage.

Well, the two chips I mentioned (DGX Spark uses the GB-10) are both a SoC, so no motherboard needed there. I don't know if that's the full explanation, but it could be a factor.

The SoC design with unified memory is generally well suited for residential use because it's quite energy-efficient, quiet and small (compared to traditional GPU-powered gaming rigs). Great performance-per-annoyance, so to say.

Mini PCs (NUC-ish form factor) are selling a lot now too, small, quiet, most people don't need expansion over what you can get from eg USB4.