AMD's overall marketshare is still low because of near-zero laptop or OEM penetration but their share of retail CPU sales (i.e. the only people that buy motherboards) is nearly 50% according to every recent sales report I've seen.
ASUS has literally dozens of current motherboards with PS/2 ports, including 5 Z370 models. I really have no idea where you get the idea that they "dropped it completely".
It looks like you got the only mono from Asus that comes with a PS/2 port other than the Apex: https://www.anandtech.com/show/11860/z370-motherboards-asus-...