Shared libraries

Except that TIFF has rich metadata support that PNG doesn't, including a whole set of private tags, and it's likely anyone still using TIFF is depending on one or the other. Changing to PNG is likely going to involve exporting that data to some format, and fixing up your system to import that data alongside the PNG.

And what's stopping someone from making a local copy of libpng and removing, say, Adam7 interlacing from it? "Nobody uses it", and it's more code surface. TIFF is a little weird, but if you support a file format, you should support it as far as reasonably possible, not randomly turning off features you're not interested in.