I'm not sure I follow your thinking here. Someone alienated to the point of non-participation, by definition, doesn't vote. They can't be relevant to measuring vote share.

The mere fact that parties are able to change their positions ensures that they will. They want to live. Policy positions form an infinite-dimensional space, unlike a conceptual one-dimensional beach; it will always be possible to divide voters in half by some combination (actually, many different combinations) of positions.