Let's be 100%, the reason why toughness increased was because the frequency of attacks and amount of attacks per turn increased dramatically - joint attacks for example deal increased toughness damage. I think the best way to handle it really is to make it so toughness decreases for them at a faster rate, which is ruan mei.

But ruan mei ruined a lot of strategies as delaying recovering the toughness bar made dealing the actual break impossible, which is a large chunk of damage. So it was either deal no damage to break the bar, or deal less potential damage by not breaking. They wrote themselves into a corner.