The effective rate for 50i or 60i video, in terms of motion fluidity, is 50 or 60fps. Just because it's half the resolution per field doesn't mean that it's not a "frame" of video in abstract terms, just that it's not a "frame" in the jargon (because fields by definition are half a frame).

It is impossible to decimate a video from 50/60i to 25/30p without losing half of the motion fluidity, even if the properly interlaced source video is technically 25/30fps.