That's all nice and great and all, but there's a huge problem in that logic. If you have the power and authority to disallow your vassals from engaging in war with each other, why do your own vassals, wo don't have the power to wage war, suddenly can allow wars for their own vassals?
One major principle of delegation is that you can't delegate more power than was vested in you. If you don't have the right to do something, you most certainly don't have the right to authorize actions that lie outside of your own limits.
But let's assume for a moment that you lose control and your subjects walk all over you. Ok, can happen. But that usually constitutes a crime, a transgression against your and, more importantly, your liege's will. You have the right and the obligation to sanction such behaviour and, if you prove that you're not up to the task, your liege has the right to intervene and, what's more, revoke your position for your failure and give it to someone who's more suited for it.
What we got in CK3, however, is that the liege not only loses all form of control, but also has to watch as their realm burns, with no recourse. Even retracting a sub-vassal, who's on the brink of unseating your direct vassal, causes tyranny. Or, in OPs case, when you land your heir and he's about to get beaten by a vassal. You can't help your own family member, with whom you're allied, in a war against their own vassal; what are alliances for, if not helping each other in a war?
What we need are realm laws that apply to all subjects.