JPEG XL on the other hand supports up to 4099 color channels, a bit depth up to 32 bit per channel (technically up to 64 bit, but this currently isn't used), supports HDR natively, can use splines to compress elements like strands of hair, thin tree branches or line art that are typically hard to compress with DCT, supports patches for compressing repeating image elements, supports thermal, depth and alpha channels, supports losslessly recompressing existing JPEGs saving about 20%, supports CMYK and spot colors for printing, supports layers and selection masks, supports storing raw camera sensor data in bayer patterns, etc.
WebP is just a web delivery format, JPEG XL was designed to support many uses cases like web delivery, medical imaging, raw camera sensor data, authoring, multi-spectral imaging... the list goes on. if we support JPEG XL now, chances are it'll be quite a while before we need a new general purpose image format because JPEG XL covers so many current use cases and was designed to accommodate potential future use cases as well.