A decent proportion of the .JPGs and .GIFs being rendered on sites I visit are actually now .WEBP files with the headers set. So I'm seeing lots of people already making this translation invisibly. You only notice when you try to save the images.
Or when you notice quality loss because the original JPEG has significantly higher quality than whatever it got transcoded to.
Seriously, people, stop transcoding to "newer, better" formats at lower quality. Just stop. I should never have a better experience if I set up an older browser that doesn't accept your newfangled stuff. Either manage to get the quality right (easier said than done) or just don't transcode stuff.
(This is much easier to get right if you are also shrinking an image.)
It seems to be pervasive for people to grossly overestimate the savings they can get, looking e.g. at how MPEG-4 TV channels are generally lower quality than MPEG-2 channels. And the biggest HEVC torrents are at most less than half the size of the AVC torrents, usually only like a quarter the size.
