Tell that to the wives of Henry VIII.
Unless a ruler is stark raving mad, there's a reason why anyone would be dealt with harshly (executed, banished etc...). New characters would usually jump at the opportunity for elevation of their status (becoming a feudal lord, marrying a ruler etc...) and be careful not to repeat the mistakes of their predecessors.
CK2 already completely favours benevolent rulers, you can't be a tyrant and have everyone be quiet due to fear of you. You can only be kind, just, peaceful, lawful and benevolent if you want no rebellions. You already can't execute or banish traitors with no penalty (why?). Raising in rebellion was the worst kind of offence back then, with those who did ended up tortured and quartered. In the game, you can take one title from them, and only from the ruler who rebelled, not from those involved.
Allowing tyranny to be remembered by all characters, new and old, you're giving people even less options to manage their realm. Banishment would be impossible because -140 would ensure that you would have constant rebellions every day until your ruler died. Changing that would require complete rework of the banishment mechanic.
Also, even with the most tyranical rulers, as soon as a place was opened, other people jumped at the opportunity and actually tried to keep on the good side of the ruler instead of rebelling
