when you go to end a flight it should give you the option to save the replay. I dont know how the current "Instant Replay" feature works although it cant work the way I want it. Otherwise there'd be replays online. It only has to record input from the user, and seeing how most aircrafts are on autopilot a lot of the time it wouldn't make the file sizes really large.
The reason I think this would be a useful addition to the community, is that good pilots could upload them taking off at such and such an airport in bad weather and landing safely in a destination. It could be an opportunity for novice pilots to see how to fly a plane properly. I think so anyway.