Having learnt standard notation one can easily read music composed for any instrument, and even compositions from a different musical culture (e.g. makam), whereas with this system one's limited to western-music piano pieces. Besides, standard notation is not hard to learn, in fact it's only as hard as learning the alphabet, which kids do in the school, and I'm yet to see someone coming up saying, hey this alphabet is too hard, better kids explain themselves with pictogrammes and emoji, which convey the ideas more directly and easier to recognise.