Safari was the least of it - most image processing tools didn’t support it (e.g. PhotoShop got support last year, Microsoft Paint was the year before) or you had to do things like recompile the versions of common tools to add support (again, better now but it takes a while for support to spread through Linux distribution releases), and now you have more security exposure. That was a lot of hassle for very modest compression gains.
AVIF has gone better because it wasn’t based on a video codec which was never really competitive, was developed collaboratively, and didn’t have feature regressions from JPEG. As with tool support, that last matters a lot at many organizations because the edge experience tends to decide things - even if 95% of your usage is boring 8-bit 4:2:0 the institutional memory tends to be shaped by the times you hit something which can’t be used without more work. If it compressed as well as AVIF, more people might have decided WebP was worth it but since it only marginally outperformed JPEG the case was never that strong.
Part of what I meant by “shipping the prototype” was this kind of stuff: someone at Google wanted to find another use for the On2 IP they’d purchased so they tossed it into a 20 year old container format and shipped it. As with WebM, the benchmarks were fast and loose which meant that anyone who replicated them saw substantially lower performance, which is another great way not to build confidence in your format.
