Brotli reduces their compute costs, whereas JPEG XL reduces storage which at least historically is cheaper than compute?
From a browser point-of-view, the real questions are about data size (if you can get the wireless interface back to a low power state, you’re winning on power benchmarks), about decompression cost (again, power benchmarks), and decompression time (for performance benchmarks and user-perceived performance).
Storage and compression compute costs aren’t really relevant — only insofar as if there’s very little adoption, the security risk of exposing the code is there but isn’t providing user benefit.