Yes, I think this is my preferred version of the scenario as well - and I love your wording of his explanation, so perfectly casual and dismissive, stating his reasoning matter of fact and expecting her, being a good little slave, to understand and agree.
....is just a perfect statement of the situation, especially since he's saying it to her directly, that re-affirms the bottom line of the master/slave relationship. She is at the end of the day just a piece of ass to be bought, used, and sold, and she is at the end of the day replaceable since he'll just buy another slave when he's ready to amuse himself with a new fucktoy.
I also think it would sting, especially if she has been his slave for a while, which adds to the emotion of the scenario and makes it even more interesting. Even a slave who is rational about her status would develop complicated feelings about the man she performs sexually for probably every single day, a man she has done things with she never did with any man she chose to be with as a free woman....perhaps because he treated her decently, perhaps because of how he behaved or things he said in the throes of passion....even if she rationally knew she just a slave being used by her master, hearing aloud from him so bluntly and casually that she was never anything more than a 'fuck bunny', one he'll simply replace when it's cheap enough, is going to hurt.