You don’t get partial credit for an incomplete extractor and the waves don’t get easier, and the extractors don’t scale very well. On SP I’ve been in a position with certain frames (Loki even with irradiating disarm) where I couldn’t clear a single extractor.
I suppose this is true for defense objectives in general though - that one moon defense node on SP, for example - Stofler. And like the other person mentioned below for mobile defense.
I haven’t done it outside of SP for a while. I know Duviri gives partial credit. Maybe normal does?