A number of small companies sold packaged S-100 systems with customized CP/M provided. I bought an S-100 bus box from "California Computer Systems" which included a serial port board and disk interface board, and they provided a BIOS set up for those devices. They also provided the source, and over a couple of years I modified the BIOS several times, adding devices and improving things generally. But lots of people bought turnkey systems like the Sol [1] and IMSAI [2] and never touched the BIOS.
My first computer was put together from pieces, but at its heart was a CCS case, backplane, and power supply. It was a happy medium between totally geeked out (IMSAI) and dumbed down (Sol). Of course I wrote my own BIOS from scratch.
I do wonder if there is room for a solution like the S-100 bus these days. That is perhaps the one thing that irks me about todays computers, that the motherboard is dictated by the CPU socket soldered on.
The capabilities built into today's chip sets and motherboards is just insane. This weekend was the first time I plugged a board into a PC in over 10 years, to add USB 3.0 to a system too old to have had it already. The last PC I put together was based on a mini-ITX motherboard, simply because I knew from experience that I wouldn't need anything more expandable than that.
I don't think it'd be easy considering the speeds you expect from the various buses. You could put everything on a PCIe card and let it drive a backplane through a specialized connector, but that wouldn't be like S-100.
A couple weeks back I was thinking about building a backplane that would allow a bunch of BBC Microbits to cooperate. That, too, would not be like an S-100 bus.
[1] http://www.oldcomputers.net/sol-20.html
[2] http://imsai.net/