Amazingly patent-encumbered, though.

Yeah. JPEG-XL is a really good replacement but it seems like it's getting nowhere. It's been years and the adoption is still quite underground.

There is some movement.

Google Research recently committed to help implementing it, in Rust, for inclusion in Firefox.

https://github.com/mozilla/standards-positions/pull/1064: “the team at Google has agreed to apply their subject matter expertise to build a safe, performant, compact, and compatible JPEG-XL decoder in Rust, and integrate this decoder into Firefox. If they successfully contribute an implementation that satisfies these properties and meets our normal production requirements, we would ship it.”

Oh that is real good news!

Not really, it's a weird hack (using interframe compression from video compression) that worked fine as a stopgap, but JPEG-XL is leagues better.