Anyone using a wireless mouse or keyboard would use the USB port with the interface adapter. Only if hard-wired, would a keyboard or mouse use the PS2 port. Range of operation is limited with hard-wired connections. And most users prefer not to see a tangled mess of wires on the desktop.
And most keyboard and mouse offers come with the USB adapter.
While each USB controller manages two ports, you can connect up to six devices on each port.. with a powered hub. Yes, response will be slower with more devices on one port. But the keyboard and mouse don't have a very high data speed, as do storage devices.
Some motherboard designs already have omitted both PS2 ports.. and have only USB/LAN ports. And some designs do not have any serial ports or printer port!! The BIOS has been set to have the 'legacy' USB feature enabled as default.
Then too, with computers having a "home-theater" intent, a wireless link is preferred.. as the computer would be next to the TV monitor.. or built into it!
F@H.. to solve mankind's maladies.. in our lifetimes!