[wolrah]

The question there would be, it has that advantage but at what cost?

I'm not incredibly familiar with sixels, but my basic understanding is that they're in a format that was convenient for use with dot matrix printers and were later adapted to terminal use with color support and such. This means that interacting with them is not going to be straightforward for developers familiar with any sort of modern graphics APIs on either the software or terminal ends.

If the main advantage is compatibility with old terminals that only really matter to a niche subset of retrocomputer enthusiasts I'd argue that any efforts to add graphics to the Linux terminal should be focused on a more modern design such as the base64-encoded images supported by a few terminals.