Sudden change in the area of research
Speaking as a combinatorist, you'll be better off being open to both subfields from a knowledge point of view.

Remember: what you choose to write your thesis on does not determine what you'll study for the rest of your life. Especially when we are talking about two subfields so closely related.

If you want to be at a university the state of the fields now matters pretty little, comparatively to the date of the fields in 10-15 years. If you want to enter industry, it's unlikely that the subject of your dissertation matters much at all (field, sure. But subsubdiscipline? Not at all).

Listen to your advisor.