1. Lossy: AVIF is vastly more efficient at any fidelity target with reasonably modern encoders (libaom, SVT-AV1). Even proprietary WebP encoders are more efficient than libjxl. More efficient meaning better quality at the same size. This is provable even beyond modern metrics.
2. Lossless: JPEG XL is ~10-13% better than WebP here, but can be >6x slower to decode. What is the point of saving 10% bits when the client pays for it? There are lossless codecs more efficient than both that are faster to decode than both, too.
3. JPEG Recompression: Same deal, the increased decode time often erases the time-to-display gains (even on modern devices) - not just me either, this was proved empirically in the previous HN thread on a Pixel 9 Pro.
4. AVIF can have as many progressive decode passes as you like, and they aren't just "layers", they actually work together.
There are strengths, like 32-bit float precision and support for a lot of layers. I'm just lost on what the criteria for including a new codec in the Web is at this point. Google was heavily involved in JXL's development so I understand their incentives exist, and the "all-in-one" codec argument, but other than that, I'm a bit lost.