There are modern systems that still use this sort of modular backplane design. MicroTCA and PXIe are the two major ones, both providing at least power and a PCIe bus on the backplane. MicroTCA supports carrying Ethernet, SATA, and PCIe on the backplane. However both of these are more intended for industrial computing. I could see a consumer variation based on MicroTCA where the chassis has a pre installed controller and backplane. The backplane would be configurable based on the CPU you are running. Higher end CPU would allow for more PCIe lanes meaning more slots, lower end would only allow for a few slots. The computer part is provided by a heavily packaged SBC containing a processor and memory. Mass storage either sits on the SBC as m.2 or as a chassis card connected via SATA or PCIe. You'd have a GPU card connected to a x16 slot (up to two of these for a large chassis). High end sound card sits on another slot. Various interface cards sit on others. You could even drop in an FPGA accelerator card.

This would all probably be too expensive for your average consumer though.