JPEG XL had so much going for it. Kinda sad it was killed off just like that.
It wasn't killed off. Support was removed from Chrome, for what appears to be rather spurious reasons, but practically everyone else are busy implementing it.
Sadly removing support from Chrome is effectively the same as killing it off. And the reason is Google wants people to use webp instead.
While I'm not the biggest fan of WebP, using generation loss as a metric wouldn't be an indicator of a real world scenario. I can't think of any actual instance where an image needs to be re-encoded, say, 10 times, let alone 100+ times.