Lower framerate without re-encode
No. The video can be slowed down, but frames from an inter-coded codec stream (like H.264) can't be dropped without re-encoding.

Update:

The command below produces a file that plays in Potplayer and ffplay but not in VLC. It works for me on MP4s, Should work on MOVs. Doesn't work with MKV (you should be able to remux to MOV/MP4 and execute).

ffmpeg -discard nokey -i input.mp4 -c copy -vsync 0 I-only.mp4


Timestamps are retained. So each I-frame shows as a freeze frame till it's time for the next one. There may be a way to make this playable in VLC, but I'll try when I have more time.