The scenario I'm thinking of is sending a photograph by email. Let's say Sally has just had her second baby, and wants to send baby pictures to her mother, the baby's grandmother. Neither Sally nor her mother are computer professionals, and they don't want to care about image formats. Sally wants to upload a file from her camera, attach it to an email, and send it. Her mother wants to click on the email and see pictures of her new grandchild. Neither one of them wants to have to convert the image themselves.
Let's say it's a few years in the future and Sally's camera produces JPEG XL files by default. (Which I don't think would happen if browsers didn't support it, for exactly the reason I'm about to articulate). If Sally's mother's browser doesn't support displaying JPEG XL, then although Sally (who uses a different browser than her mother) sees the picture in the email when she sends it, her mother writes back an email saying "the picture didn't show up, can you resend it?" And they spend a few back-and-forth emails before finally getting tech support from a friend who helps Sally convert the picture to old-school JPEG so her mother can see it.
If that's the experience for end-users, cameras would hesitate to switch to JPEG XL as a default format. Because nobody wants a hostile review from a user saying "My mother couldn't load my baby pictures, what kind of lousy camera are you making?"
So even if it's not the best format for the Web, I'm still persuaded that having universal browser support for a format intended for photos is an important factor, and devices like cameras will not switch to it as a default format until manufacturers are certain that browser support (and OS support, and electronic photo frame support, and so on) is nearly universal.
None of that takes away from the argument "Other image formats are better for saving space on the Web", which is the main thrust of your post. But I do disagree with the last line, "I'm just not personally convinced we need it in browsers any time soon." I believe that if browser adoption is lacking, adoption of the format in places where it makes lots of sense (like cameras) will also be slow.
What are your thoughts on that argument?