In Yevamoth 3b, we learn that the reason for exempting fourteen women (of the fifteen enumerated in mishna 1:1) and their co-wives from yibum (levirate marriage) and chalitzah (the decision not to do levirate marriage) is due to a hekesh (textual comparison) with the case of a man's wife's sister.
מה אחות אשה מיוחדת שהיא ערוה וחייבין על זדונה כרת ועל שגגתה חטאת ואסורה ליבם, אף כל שהיא ערוה וחייבין על זדונה כרת ועל שגגתה חטאת אסורה ליבם
As a wife's sister is singled out in that she is a forbidden relative, the penalty for presumptuous intercourse with her is kareth (spiritual excision) and for unwitting intercourse a sin-offering, and she is forbidden to the levir, so also any woman who is a forbidden relative, and the penalty for presumptuous intercourse with whom is kareth and for unwitting intercourse a sin-offering, is forbidden to the levir. [translation from here]
The same hekesh is then used to exclude the co-wives of the fourteen women from yibum and chalitzah.
The gemara then comments that this hekesh shows that a different group of six women do not exempt their co-wives from yibum and chalitzah, by finding an attribute of the case of a man's wife's sister that is not shared by the cases of the six women, namely, that in the case of a man's wife's sister, the sister may marry another one of the yavam's (levir's) brothers, whereas the six women may never marry any of the brothers of the yavam.
My question: why do we not argue that the first hekesh is invalid, by saying that in the case of a man's wife's sister, the sister is permitted after the death of the wife, whereas in the other fourteen cases, the women never become permitted to the yavam?
It's clear that we're willing to use such an argument by dint of having done so to include co-wives of the six women in yibum and chalitzah. Why do we not do so here?