Putting aside 8bit computers, there really is two way of approaching it:
- the hardware authenticity way: bringing back ISA, IDE, Floppy and PS/2 ports on modern motherboards. With a scalable CPU, people could use their legacy hardware and software or have new manufacturers of retro hardware to replace the expensive old junk, like the X1 addon, that MPU card, or the GUS enthusiast project on this forum, etc. It's my preferred way of doing it.
- the hardware emulation way. It's what the industry did, although they really did a half hassed job. Modern BIOSes can have emulation of legacy hardware, passing USB mouse and keyboard input into a virtual PS/2 port, simulating IDE on a SATA drive, allowing to boot on a USB floppy drive. Support is really spotty and could be much better, but there isn't any technical roadblock, only a lack of will to do better than good enough. Sound is still a problem this way, but I remember reading how simulating an ISA slot via a PCI-e card could be feasible. It would allow sound card manufacturers to bring back legacy support as well. We are almost there with modern graphics cards and their support for glide wrappers, we just lack legacy drivers for legacy OSes.
I firmly believe it is possible to have a modern PC fully compatible with legacy hardware and software. Now whether a company will make such a PC is highly doubtful, but our modern PC have still the potential for being a compatible IBM-PC, if the effort and money is put in.
The real problem, and this is true for everything retro, is the display. I'm a CRT purist and believe everything look better on it and retro stuff look right only on one. But they are old and dying and no one cares about them 🙁