There are differences--you're often willing to do more work decoding a still than a frame, say--and in the first story I linked there's a quote from a Googler wondering about whether to make the image format exactly track AV1 or to tweak it. So, there's a real question.

That said, the still demo at https://people.xiph.org/~tdaede/av1stilldemo/ is really encouraging, and Apple's seen a win in doing something parallel with HEVC/.heic. AV1's already gone through a lot of optimization and IP vetting (patents caused trouble for previously proposed replacements for JPEG), and has a hardware decoder design and a lot of companies signed on. Like everyone I just have to wait and see what they do, but I'm more or less on team "ship it" here.

On the other hand the bar for still images is very low due JPEGs dominance. That's why it makes sense to use video encoders for stills even if they are not ideal, you are still going to get much better results than JPEG.