Yeah, I guess I have a longer view since our first IBM compatible PC was a 286 based XT form factor. And in households with multiple computer users, upgrades could look more like mitosis (or nuclear decay?), with some parts splitting off to form new computers and less clear lineage of one computer just mutating.
The buses I was thinking of included ISA, EISA, VLB, PCI, PCIe. Yes there were ways to carry some things across since motherboards often had a couple bus types at once. But in my experience, the older peripheral cards often just got retired as they became either obsolete concepts or totally integrated in the next motherboard. I.e. you once commonly had serial port and parallel port expansion cards, game controller cards, sound cards, disk controller cards, and basic 2D graphics cards.
Cases also got smaller because the motherboards needed less space, people needed fewer expansion cards, and also because people needed fewer and fewer "drive bays". In the early days, you saw both 5.25" and 3.5" floppy drives, CD-ROM drives, big chunky HDDs, and possibly other weird removable media drives. Now you can easily have a capable corporate-style PC with no expansion cards, and no drives other than the M.2 stuck into the motherboard.
On the external side, I can think of PS/2, serial, parallel, USB, external SCSI, Firewire, e-SATA. Some of these coexisted with USB until it became high speed enough to subsume them. With graphics there was VGA, composite video, DVI, DisplayPort. Sound had 3.5mm, coax, toslink, coax digital. Communications commonly had POTS modem, coax ethernet, twisted pair ethernet. Somewhat esoteric were WiFi and bluetooth adapters. These could be on dedicated expansion cards, integrated into sound/graphics/comms cards, or integrated into the motherboard.
There were also weird expansion cards that paired with a particular external device, like a scanner or Hercules monochrome monitor. And more unusual cards like video-capture and digital TV or radio tuners.
The PSU issue wasn't just overall wattage but different set or balance of voltage rails and kinds of internal connectors needed for powered components. And shifts like standby power/soft-off behaviors.
I also recall AT to ATX and later uATX. Earlier motherboards were massive with socketed DRAM and SRAM chips and lots of simpler logic chips all over. They just kept shrinking as everything got more highly integrated. If you ever got a surplus Dell you might have encountered BTX too, which was like the left-handed universe.
I also had a phase with two uATX cases and almost had a "two space garbage collection" upgrade cycle, shifting parts in, between them, and out. One was my desktop PC and the other a "media PC" attached to TV and home stereo.
Some folks like me had a phase of trying to accelerate the down-sizing, abandoning our ATX/uATX for things like the Shuttle XPC mini/bookshelf computer formats. This meant more incompatible chassis, motherboard, and PSU formats. For me, a computer after 2000 was case/PSU + mobo/CPU/RAM + disk. The disk was either a single HDD/SSD or small software RAID array. At one time, we needed multiple disks for capacity, but now it can just be one or two M.2 drives on the motherboard and no disk bays at all.
This also leads to periodically thinking just a laptop will suffice, and then that becomes another thing that sees little upgrade and carry forward over longer time periods...