And Mozilla waited for libwebp to reach 1.0 before making the plunge to support it in Firefox. One of the main issues was that libwebp was willing and frequently made backwards-incompatible changes to the codec in the 0.x days; Google simply didn't care about breaking images, but Mozilla cared.
Yes, that's a problem when a format is not standardized but controlled by a single entity. JPEG XL is standardized though, and backwards-incompatible changes are essentially impossible.
That said, we are aiming to reach the libjxl 1.0 milestone within a reasonable timeframe, i.e. somewhere in 2023, preferably first half.