Re: Homebrew PCs are simply becoming more common
I work at one of the specialist builders you mention, and I think that while 'PC sales' on the whole might be on the decline, gaming PCs and 'custom built for you' gaming PCs are definitely growing, hence the increased sales of NV GPUs.
There is no real growth in overall PC sales because all you need to do for most work/home uses is run a web browser and maybe MS-office (though even that is moving into the cloud), and a PC has to get very old to become incapable of doing that, so mostly the market is just replacing aged units which fail.