JXL is one of the technologies that I hope we can fully transition over, i.e. in a couple of years nobody (even non-tech-savvy people) are sharing or copying or saving JPEGs.
And it supports animation by default, so the "motion photos" will be natively shareable.
Also, recompressing JPEG as JPEG-XL gives a free boost, so simply, why not. If you're sharing something in social media or anywhere, there is no reason to not convert and have the other person receive JPEG-XL by default.
My point is less about the size and so on. But what the visual (bit depth and color gamut) and functional leap of JPEG-XL. It's a format that combines and replaces the many different use cases that different formats are doing now.
This is not enough for companies to invest heavily in.
JXL is broad use/ universal. High bit depth, wide gamut, different color spaces — from HDR displays, to web, to print to medical photography and so on. It would help the device that your viewing this on, right now.
It's not like WebP where the only focus is to reduce file size. The fact that it does this also, and also recompresses jpegs lossless, is just an addon, not a singular reason.
If you get a chance, ignore all the file size noise, and look at JXL for what it is and how it's being adopted by different industries.