I think that most people who advocate USB-C do it because they want the world to use only one cable, so they wouldn't need so many different ones.
For that matter, why do people prefer PS/2?
Myths.
1. People used to believe that you couldn't do N-key rollover over USB... Well that was
partially true, because first it took a while before the industry figured out how, and second there were some stupid bugs in some host USB stacks which made early attempts at NKRO over USB incompatible with them. And many keyboards listed as being NKRO are actually still only 6KRO over USB.
2. PC gamers look for the maximum speed, and the theoretical speed for
a single key press over PS/2 is allowed to be
slightly faster. Actually the allowed rate of PS/2 (key press per second) is a
range within which the more fixed max rate of Full-Speed USB (frame per second) lies, so ... a PS/2 keyboard could actually have a
lower transfer rate than a modern mechanical USB keyboard.
But that is still only
transfer rate, not
latency. In practice, there are many other factors within the controller that can add latency. It is likely that the microcontroller in a vintage PS/2 keyboard is much slower than a 32-bit ARM microcontroller in a modern keyboard for everything but the protocol itself.
Not that there isn't room for speed improvements in modern microcontrollers: for one thing, much USB hardware require packets to be put in a queue whereas the USB protocol is based on polling¸ so a packet may have to wait in its queue for several microseconds before the host polls it — thus being several microseconds stale when it is polled instead of being the last recorded state.
I think that a lot of the USB hardware (and some of the HID protocol itself) had been inspired by the PS/2 in the first place — where you
had to queue up input. I would speculate that the rationale for some could have been to make it easier to adapt devices to USB back in the day when USB was introduced.
And BTW, it is also possible to run a keyboard over
High-Speed USB which is even faster, but I have only seen that
done by an enthusiast.
You can turn on your computer from the PS/2 keyboard.
You can do that with USB too.
Both types of ports have to be powered to allow this, but a USB device connected to an "off" computer is required to go into "suspend" mode where it must draw only a very small current. PS/2 does not have a low-power mode.