Safari support means there’s benefit to web server support. Server support means there’s benefit to browser support in other browsers. Apple can kick start the network effects necessary to get this standard adopted.
Webp and heic are fine for web, but JPEG XL is special in that it actually has use for print-based and other ultra high resolution workflows, while also having the best path forward for migration from JPEG.
The ceiling is going to be lower than with lithium. Sodium ions themselves weigh about 3 times more than lithium, for the same +1 charge. So it’s not just that sodium is a certain number of years behind lithium. It’s that it’ll likely plateau at a point permanently behind where lithium will likely be.