> A jpeg straight from a camera is 'more lossless' than a downscaled 8-bit png that was created from it.
Well there's your problem right there: PNGs don't come from cameras. PNGs come from image generation software that produces specific, discrete pixel values. Changing #555555 to #565656 in lossless image data is no more acceptable than changing "UUU" to "VVV" in lossless text data. Unfortunately, gzip et al don't perform well on image data, so different but still lossless compression algorithms are required.
Well there's your problem right there: PNGs don't come from cameras. PNGs come from image generation software that produces specific, discrete pixel values. Changing #555555 to #565656 in lossless image data is no more acceptable than changing "UUU" to "VVV" in lossless text data. Unfortunately, gzip et al don't perform well on image data, so different but still lossless compression algorithms are required.