It's not a strawperson. JM is not saying that Wuss thinks we should kill infants just because they are going to die, but that Wuss's analysis justifies killing a person because other similar persons are going to die anyway. No one says that Wuss' argument necessitates killing---just that it justifies killing.
I think the answer you guys are missing to JM's argument is that we do, in fact, care very deeply about preventing the death of old people and infants. So, the question is again raised: then why aren't abortion haters doing anything to stop the death of embryos? We try to stop the death of persons, and since we're not trying to stop the natural death of embryos, then maybe that indicates that even pro-life people do not truly consider embryos to be persons.
However, I think the answer to this is simply that it's not a fair burden to place on pro-life advocates. Just because they think embryos are persons does not mean they should be asked to do the impossible. Look, there is no way to prevent a week old embryo from being flushed out of the mother's womb before anyone knows that it is there. If it's impossible to prevent, then there's no moral responsibility to try to prevent it. You don't get moral credit for trying to do the impossible.
So, okay, a lot of embryos die. And yes, pro-life people are not doing anything about it. But it's because they can't do anything about it.
If Wuss' argument is that the impossibility of saving some embryos lives justifies killing other embryos, then I really don't think it's a strawperson to say that we can't stop the death of some old people, so we should kill others. It's pretty clear to me that the life of an embryo that has been in the womb and healthy for over a month is distinct from the life of an embryo that was flushed out after a week. That's why we don't justify killing an old person because another old person is going to die anyway: we recognize that the death or life of those two distinct individuals do not depend on one another.