Yes... but you're forgetting one thing... human beings don't generally like doing that...
Theres actually quite a few games where you literally can parry your way to success.... the reason people don't is because they actually want to have fun.
In the case of Nioh... it actually would be better if people parried everything because atleast then they would using an obvious feature in the game rather than the many many other cheeses Exploits like the Sign Of The Cross Loop, The Daiba Washi Loop, The Paralysis Loop and The Spearfall Loop.
Dark Souls made this mistake... they actually thought the problem was Chain Backstabs...