Unified memory DGX Spark and RTX Spark laptops are already a thing :)

RTX Spark already launched.

that undercuts their core business, so it will be a defensive play at most to fend off mac and amd's local inference offerings

I'd be happy enough if Nvidia decided to launch an update to the now dated (2019) Shield streaming box, which is still the best on the market as to codec support and image quality.

Yes. The more capable SoC PCs become, the weaker the "everything has to run in the datacenter" argument gets. As more powerful SoC PCs from multiple vendors appear over the next few years, that advantage may shrink for many workloads.