I thing the endgame is lossless PNG with 16bit color components with basic compression like gzip or at best bzip2. Or a format without the weirdness of PNG 'line based loading' which is obsolete nowdays. The "expensive part", apart from the compression algorithm, being the meta data storage without kludge.
I believe 7zip outdoes bzip2 these days along every metric
And lossless jpeg-xl exceeds your idea
These days I favour lossless WEBP instead of PNG. I often save around 30%~40% of file size compared to PNG, especially when the PNG is encoded with lots of bits per pixel (i.e 48 bits or 24 bits).
>without the weirdness of PNG 'line based loading' which is obsolete nowdays
How? Line-based loading is a very simple way to exploit 2D spatial coherence. A 1D format like gzip loses this advantage for minimal gain in simplicity.