I don't agree with you that the abortion issue is swinging more towards the Dems, but I'll answer as a theoretical.
In such a case, the Republicans can make a strategic shift towards the Center/Left in the hope of picking up enough swing votes to cover the loss from the right.
I don't think it will work, but they can try.
What I suspect will happen is such a situation will be another Bush Sr. "Where will they go?" -> Ross Perot moment, giving us once more a nightmare Clinton presidency.
I posed my question as a hypothetical. I do happen to believe that the pro life movement has pretty much lost the first trimester abortion issue, but that's not necessary to my point.
I agree with your analysis, though. A political party that consistently appeals to a shrinking segment of the population isn;t going anywhere worth getting to, in this world anyway.