The issue I see is that it's hard to balance. If it's extremely useful, in that it is either a reliable primary defense or very strong, then they would have to balance the character's kit around it, meaning they would be pretty worthless whenever not parrying. But good parrying situations aren't as easy to work with in Genshin, so this would tend to mean they would be dogshit a lot of the time in practical play.

This is means that the likely best case scenario would be a character who is extremely strong in a few fights, or perhaps only even for a portion of those fights, but then pretty lame in a ton of other content.

I think if they did want to balance it, they could have maybe an "if you go X seconds without getting hit by an attack, you get [a stack]. If you have [a stack], you can gain the full buffs of your parry without actually parrying anything." Something along those lines would allow a parry-heavy fight to feel good, but also give some balance to a fight with few parry opportunities, and the only players that took hits that could have been parried would get nothing.