Well... Sounds easy, I mean, modern PCs should be able to play 15 year-old games without a problem, but then you consider that the games were made with another hardware in mind. So you have to make the PC and game "think" it's actually a completely different hardware. And that requires extensive knowledge of how the original hardware even works - which is the reason why there isn't any Xbox emulator, there's barely any documentation about it, despite it having a x86 architecture (which, no, doesn't make it easier to emulate, that's a common misconception. It makes it harder if anything).

And the Saturn, for example, was notoriously difficult to program for, so imagine emulating that. And the N64 also had some weird crap behind the hood and some devs took advantage of unorthodox methods to run their games, making everything much harder... it's technical and complicated, and honestly, I too don't get it that well.

That's obviously the best solution. PC can already emulate GC/Wii/PS2 perfectly with just a few minor issues. Though there are people that just don't like gaming on PC (even with a controller) for some reason, and would rather have one console to rule them all.

And no, a PC can't play PS4 games, and won't be able to do it for at least 10 more years, and that's being extremely optmistic. PS360 just took the first steps, a full 8-9 years after their releases. Hell, the original Xbox doesn't have a working emulator yet, and emulating Saturn/N64 is still tricky to this day (Rogue Squadron is unplayable on a regular emulator, for example. You need a pretty powerful PC to run it).