I guess I can agree with this, though I do think that the relation gain/loss should be either a net increase or net loss based on the lord's personality- as it was in Warband.
The best way of doing it, assuming TW was willing to put effort in this area, would actually be to have three steps.
Attack: Attacking lords makes you lose relations.
Defeat: The more cautious a lord is, the more relation you gain for defeating them in battle.
Release: The more honorable or merciful a lord is, the more relation you gain for releasing them.
A lord with an honorable, merciful, or cowardly personality will appreciate being released and factor it into their future relations, and in the case of cowardly lords, will fear and respect you as a superior warrior. So defeating and releasing them should be a reasonable net gain.
A lord with a dishonorable, cruel, brave personality will despise being defeated, and doesn't play by the rules anyway so sees no reason to be nice just because you were nice to them. So defeating and releasing them should be a slight net relations loss (but not as much as if you had imprisoned them).