That's a more interesting question than I think you realize.
As I read your statement I reminded, again, that the reason you fight against the idea of personhood is because, deep down, you realize that if personhood is ever granted, right to life supersedes all those other expressed rights.
Your question actually encapsulates the whole problem underlying the Pro-Choice movement in my mind. The Pro-Choice movement doesn't fight to deny human rights to the unborn because there is a compelling argument to make for the claim that the unborn aren't human, far from it, instead they fight to deny the unborn human rights so that they can legally excuse killing them.
This simple truth was my breaking point nearly 30 years ago. I was in a discussion with fellow Pro-Choice people and we were discussing this very topic (personhood), and how the other side had called abortion a "holocaust" and how silly and hateful the argument was. Someone made the flippant remark that abortion is no different than killing a cow, and people have no problem with killing a cow. I'm not sure why it hit me so hard, but at that point a voice in my head shouted "But they aren't cows!" at which point I realized that there really was no sound scientific argument that could take the unborn human and explain them away as a heard animal, or even less in the minds of many of my fellows pro-choicers since many of them were vegans and would never kill an animal.
At that point I saw their arguments for what they were. They weren't seeking rational truth, they were rationalizing death. The whole anti-Personhood argument was suddenly too uncomfortably close to a genocide defense for me.
