I think there are more grey areas in the classification. Just take recreational weed use, which is as legal as smoking cigarettes in some regions but illegal in others. Where does the drug classification start? It's not as clear to me as you seem to indicate.

Also, the content label doesn't tell the proprietor of a cinema who can come and see a movie. Do they have to look up a combination of letters to see if a 14yo can go see It? How about something like Trainspotting where there is violence, foul language, and lots of hard drug use. A VLD rating doesn't indicate anything. But for me the movie isn't R because for all the pearl clutching it may cause for some people it's a good promotional film to stay away from heroin.

You can run your system parallel to the existing one. And some of the streamers do that, I think. But for legal reasons, in the US as well as most of Europe, you cannot not give an age limit as well.