WebP was the classic Google “ship the prototype” move - they were hoping Chrome could muscle it through but it delivered only modest compression improvements (10-15% real world - the marketing papers promised 30% based on comparisons to unoptimized JPEGs) but it was missing features and had very primitive software support, making it harder to produce, deliver, or share (when Facebook switched, a common complaint was someone downloading a picture and finding it didn’t work in another app or when they sent it to a friend).

Very few sites pay for so much outgoing image bandwidth to make that compatibility cost lower than a 15% savings.

I switched to WebP about a year and a half ago. I’d been watching for a long time and it had finally reached the point where support was universal enough that I could not publish a JPEG.

WebP has the big advantage that the quality setting is meaningful, you can set it at a certain level and then encode thousands of images and know the quality is about the same. This is by no means true about JPEG, if you are trying to balance quality and size you find you have to manually set the compression level on each image. Years back I was concerned about the size of a large JPEG collection and recompressed them which was a big mistake because many of the images were compressed too hard.

In 2023 I think you can just use WebP and it will work well, my experience looking at images is that AVIF does better for moderate to low quality images but for high quality images it doesn’t really beat WebP.

"switched to"? There's no full switch to anything.

Webp has been around for over a decade. AVIF will probably get adoption as a faster rate in my estimation.