In the 80's PCs and software ate the world, and one of those reasons (apart from DOS/Windows) was that anyone could put something on a PC expansion card which more or less directly spoke to the CPU and extend the hardware.

So the PC acquired a very diverse set of cards and was able to do heavy processing in a very diverse set of roles - from medical equipment, cash registers, MIDI/music composition, graphics, publishing, sound, networking, etc. No business was untouched by the PC in the 80's--the ability of anyone to make hardware for the platform was a significant part of that being as cheap as possible.

PC hardware seems to be slowly moving towards being like a cell phone - everything onboard/builtin, ports picked by the manufacturer (you will get 1 USB-C port and like it), and if you want to add something, your primary option is a USB-type port of some sort - with its own world of firmware, controllers, etc.

Look at a smartphone and a modern ODM-designed cheapo laptop motherboard - they're very similar.

If motherboards stop being expandable it might mean the PC hardware ecosystem isn't going to be able to innovate except at the behest of high-capital firms like Intel, AMD, etc. That might be OK if we are truly at the zenith of what's possible with a PC, and really want to give the keys to the kingdom to those firms.

And who knows - I know somewhere in there USB4/Thunderbolt on some level is able to move PCIe traffic, so maybe it won't be so bad.