Point was more: Get rid of this bloody Download a JPG, do some Stuff &
Upload a 3 Times locally saved JPG again […]
I'm afraid I did not made my point clear enough. With all respect to your enthusiasm, but the scenario you describe is exactly what your suggestion will not improve. How could it? We can not control what people do on their local computers.
Even worse: FLIF is not even needed for the scenario you describe. We could just convert all JPEG to PNG. But this will not happen for the reasons collected in this thread.
those images should never be saved as JPG in the first place.
Sure. Go and encourage people to upload RAW. That's very much welcome. But converting their JPEG after they made them will make many scenarios worse. Even including the one you aim for: What you want is to allow people to download an image, open, edit and save it without ever thinking about the file format. FLIF can not do that, yet.
Best Thiemo