No, I’m saying that JPEG XL can perfectly represent old JPEG/JFIF data, so on the server side you can store all your image data once and more efficiently, and still support old clients without any lossy cascade or the CPU load of having to re-encode. That is what is meant about it offering backwards compatibility.

It’s “compatible” in that it can represent old JPEG/JFIF data more efficiently and in less space, and the transformation to JPEG XL and back to JPEG/JFIF is lossless (in that you don’t lose any /more/ quality, you can get the same bits back out) and quick enough to be doable on-demand. You could, for example, re-encode all your old photos on your CDN as JPEG XL without loss of quality but save a bunch of disc space and bandwidth when serving to modern browsers, and translate dynamically back to the old format for older browers, all with no loss of quality.

Does CrossOver Office support a version of Excel which ticks your boxes? Are you in a position at your workplace to move them off Excel? I honestly have never found an organisation where they were using it for what it was meant for, often being used in place of proper tools - databases, issue management systems, requirements tracking, etc - where the better tool would be better and cheaper.

But, like JIRA, the people who make decisions only know one tool, and that tool is a hammer.