AVIF is often unsupported, but in recent years, most sites seem to accept WebP. There was certainly a time in which most sites rejected WebP, and there are a handful of websites that still do have issues with it today, but it is certainly not “half the internet” at this point.

EDIT: Your main point is definitely true, though. It took several years to reach this level of WebP compatibility, and its still not perfect. JPEG XL compatibility adoption will unfortunately likely be just about as slow.