Think about it - why do professional photographers bother with shutters or try to time the perfect shots? Why not just constantly shoot video and pull frames out of it later? Because it would look like crap.
This wouldn't be hard to imitate. Record the live preview into a 3 second buffer. When the user presses the shutter, switch into still mode, take a real photo, wait 1.5 seconds, and then write both the buffer and the photo into the same archive. On playback, by default show the normal photo, but on whatever UI gesture, play the first 1.5 seconds of the video, then the photo for one frame, then the next 1.5 seconds of the video.
Only barrier I could see to re-implementing this on other phones is long wait times while the camera switches modes, during which information is lost.
Photographers are doing this.
https://news.creativecow.net/story/879117
>"RED cameras allow Inez and Vinoodh to simultaneously capture high-resolution images for their photographs along with their artistic videos"
You can get to comparable image quality, but it's going to cost you.
Two separate files tied together by the UI.
That "Live Photo" will still be a normal jpg