If you look at slide #14, you'll see Opera was an early adopter.
Firefox published a "No" blog-post in 2013, and Safari removed WebP support from Sierra preview in 2016, eventually adding it back in 2020. Stuff happened.
And yes, when WebP was created there was a real, non-incremental, need for a Web-oriented image format. Nowadays, it's just incremental improvement on this idea for browsers.
AVIF is equivalent in decoding complexity, even in software. It's not order of magnitude different.
Encoders OTOH can be as slow as you want them to be.
iirc, back in ~99, Unreal Engine was doing dithering on the u/v coordinates (!!) for texture mapping instead of bilinear interpolation. They used fixed Bayer matrix.
This was quite faster, visually pleasant, and was adapting nicely to viewing distance.
This 'new' WebP format is not radically different, but just pushes the same use-cases further
(The web. Not archival. Not camera capture.), sticking to simplicity and usefulness.
10 years is a long time since WebP-v1, and techno has evolved since. We can go much lower in bitrate for still-good-enough quality, and that's what we're interested in exploring.
AVIF was designed starting from a video codec, similar to WebP-v1 in the early stages, and this has some drawbacks (that later we needed to correct in WebP, that was confusing).
WebP2 is aiming at images-on-the-web right away, and not as an afterthought.