The thought experiment functions as an informal reductio ad absurdum of the argument 'Fetuses are people. Therefore abortion is immoral.' or 'Fetuses are conscious. Therefore abortion is immoral.' That's all it's doing. If you didn't find the arguments compelling in the first place, then the reductio won't be relevant to you. Likewise, if you think the whole moral framework underlying these anti-abortion arguments is suspect, then you may want to fight things out at the fundaments rather than getting into nitty-gritty details like this. The significance of the violin thought experiment is that you don't need to question the anti-abortionist's premises in order to undermine the most common anti-abortion arguments; they yield consequences all on their own that most anti-abortionists would find unacceptable.

That is the dialectical significance of the above argument. It has nothing to do with assuming that everyone found the original anti-abortion argument plausible. An initially implausible argument that's sufficiently popular may still be worth analyzing and refuting.