...and because many people want abortion to be legal

don't forget that part

> When any law can be overturned, there is no law.

This doesn't follow. Right now, in the US people drive on the right side of the road. They could decide, at any time, to switch to driving on the left side of the road. But that wouldn't mean that there is no laws - it would just mean the law would be something different.

The US can decide that life begins at conception, and the right to abortion is not superceded by the right of the fetus to live. Or they could decide the opposite. In either case, the law can be made clear so everyone understands the law, and abides by it. The rule of law can hold in either case, or even if the law changes from one to the other based on demographic and religious necessity