If it was actually 2x more efficient then that would be a very strong argument that I believe would prevail. But I'm kinda surprised so many people are so thirsty for a 10% improvement for natural images at qualities above what most websites serve. Which might even be reduced if AV1 encoder folks ever start caring about that quality range, given that it's way above what's relevant for video...
Give it another couple years and maybe we'll have another format 5-10% better than JXL, at which point I hope everyone starts getting angry that format isn't included in browsers... (actually VVC intra should be at that performance level today, not that it has any chance of becoming the basis of a web image format...)
> JPEG XL could be twice as efficient as AVIF everywhere…
This isn't true if you're talking about encoding size efficiency. See the "Quality" chart at https://storage.googleapis.com/avif-comparison/index.html
Even JPEG XL cheerleader Cloudinary says, "JPEG XL can obtain 10 to 15% better compression than AVIF" overall, which isn't a sufficient incremental benefit. https://cloudinary.com/blog/the-case-for-jpeg-xl