As hym said there was a TC capture system already. I think people just felt worse when their TC was captured and taken away from them benefiting the enemy even more. It was actually more of an incentive for sieging because you could deconstruct the buildings for scraps, or keep and use them.I think the emphasis on "value" of keeps is what needs work. Keeps need functional, social, and political value. Being able to capture keeps shouldn't devalue them and if it does, I feel keeps themselves don't hold enough value. I do enjoy this discussion!
But as long as a keep is something of value people will attack you for it. Then people who react really negatively to losses will be in the same situation that they were in before, but perhaps to a lesser degree. I doubt it