Yeah but this is quite at a higher level.
As you say, they already did that since their ecosystem is a complete mess, with a shitillion of formats, some of which being a pain to manage e.g. Topaz—details from an engineer were published in a forum a few years ago but obviously, the thread has vanished since.
Now, if say you download the file on your Kindle Voyage then sideload the file on your Kindle Fire, it will work… but images will still be grayscale.
Today, it’s mainly about images but who knows what they’ll adapt to devices in the future? Layout? Typography?