The "a fetus is a person" attempt to frame the abortion debate actually seems like it would weaken the norm against killing innocents. Most people agree with the rule that it's generally wrong to kill an innocent person, which is a relatively clear bright-line rule. If pro-abortion people can just say "well, a human fetus doesn't count as a person so the rule doesn't apply there" then the rule against killing a person remains relatively clear and simple for them. But if they have to count a human fetus as a "person" then the rule against killing a person becomes messy and complicated for them - they have to say "well, it's often wrong to kill a person, but there are various exceptions and factors to weigh."

Anti-abortion people might like having the abortion debate take place on those grounds, with a human fetus counting as a "person" by definition, because of the rhetorical advantage it gives them within that particular debate. But for the broader goal of establishing shared support for the "sanctity of life" it is counterproductive to cast the abortion debate in those terms. If you use a dictionary to remove the flexibility/disagreement in defining the domain where the rule applies, then that flexibility/disagreement gets shifted into the content of the rule.