That's not descriptive ethics though, that's regular moral philosophy.

Fair enough. I was trying to express the following point: One of the advantages of descriptive ethics, especially if done via a well-designed questionnaire/survey, is that participants will engage in some moral reflection/philosophy, potentially illuminating their ethical views and their implications for cause prioritization.

For the 2nd point, moral compromise on a movement level makes sense but not in any unique way for population ethics. It's no more or less true than it is for other moral issues relevant to cause prioritization.

I agree that there are other issues, including moral ones, besides views on population ethics (one’s N-ratios and E-ratios, specifically) that are relevant for cause prioritization. It seems to me, however, that the latter are comparatively important and worth reflecting on, at least for people who spent at most a very limited amount of time doing so.