Well, for PNG, Apple have shipped exactly that. It was something they could unilaterally do without e.g. having to get all of the major browser makers on board.
Apple's PNG encoders (i.e iOS dev SDKs) produce multi-thread-friendly PNGs and their decoders (i.e. iOS devices) reap the benefits. And unlike trying to introduce a new but not-backwards-compatible-with-PNG format (JPEG-XL), other PNG decoders can still decode these images. They just don't get the speed benefit.
> Any system that has to take arbitrary png as input cannot rely on MT decode being an option, etc.
It's forwards compatible too. Apple's PNG decoder will use MT decode if the PNG image was encoded with MT metadata, but Apple's PNG decoder can still decode arbitrary PNG (using its pre-existing single-thread code path).