The Rosh explains why the gemara did not ask the question according to the rabonon re the difference between 30 days deoraisa (where he would bring korbonos, etc.) and 30 days derabonon. However, why not ask according to the rabonon after the yom melos when there is no gezeirah and he is not soser 30 days vs. 30 days betoch melos.
dmartin, raanana,eretz yisroel
The Gemara is explaining the Mishnah which says "Im Ad she'Lo Gilach Soser," and the question is how much (is it Soser). We are clearly not discussing a case which is not Soser,
D. Z.