Commentary for Kiddushin 119:10

The Talmud notes that the logic should work in the other direction as well. If we say that she cannot have yibbum in one case then we should say the same about the other case. In other words, they are both similar, so we should not let he have yibbum in one lest she come to perform yibbum in the other.

In the end, the Talmud admits that while it is preferable for her to have halitzah in a case where he says “from today and after my death” if she does have yibbum it is not a problem. Again, in this case there is no real doubt to R. Yohanan that this is not a valid divorce. It is only a rabbinic prohibition that she should have halitzah and not yibbum lest people allow the woman divorced by a man who said “from today if I die” who most certainly may not have yibbum.