I think in today's machine (be it PC, Mac or even Smartphones) the reading-and-transform of a png asset for the GPU is hard coded in the processor : so VERY fast. It works well.
There is no hardware acceleration for PNG in any consumer device that I know of. And there is such thing as VRAM usage, if you decompress PNG to bitmap it would use 3x-4x more VRAM than BC7. If you decompress it from PNG and then compress to BC7 it would increase loading times and basically destroys all point of using PNG instead of lossy compression.
