I'm using interlaced video because: in 50 fps picture move became more smooth and there is no motion blur in kdelive which may help to improve 25 fps video.
When I'll try change size of my video from 1920x1080 to 1280x720 - output picture became awful (and it have no interlace after all). I think that it is not very hard to make correct resize technique which will save interlacing on resize (and save 50i fps)
Algorithm (as I think) should make something like this:
1. We are want to resize 1080i to 720i
2. 1080i frame separated in 2 half-height progressive frames 1920x540
3. Each frame resized to 1280x360
4. Merge two 1280x360 frames into one 720i frame
Second needful thing it is deinterlacing and interlacing features. I mean sometimes maybe better deinterlace 1080i into 1080p(50 fps) and than resize it to 420p 50fps, or edit project with multiple videos in 1080p(50 fps) mode. And sometimes after editing video in 1080p 50fps mode you will need to produce finally 1080i output.