"You'd waste a few KiB on encoding skip blocks" This tells me you don't know how codecs work... skip blocks are not expensive to code, they are very cheap.
Jake's blog post is outdated by the way, the progressive functionality is much more advanced than it was at that point in time because Julio worked on it.
Laughable.
My conservative estimate is that a few-MP image with all but a smallish region encoded using skip blocks will spend a few KiB on that. This is very expensive compared to sending only a bounding box, hence it is unattractive for purposes of updating small regions with a whole-image layer.
I am glad to hear AVIF progressive has improved. But note that my original comment was: I find it misleading to call AVIF's "up to four passes" "very flexible". I believe that stands: contrasting the flexibility of (purpose-built) JPEG XL vs. the fairly strict limitations (inherited from video) of AVIF, I am astonished anyone would still call the latter "very flexible" by comparison.