RE: " Why won't we just all switch to PNG"
First, it's not nearly as well supported as either TIFF or JPEG.
In my case, when I'm scanning documents, I am almost always going to save
them as PDF files rather than as [explicitly] either TIFF or JPEG [although,
internally, my PDFs created from a scanner are JPEGs]. I'm not even sure
that using an internal PNG format is easily possible.