A lot of people don't understand how anything like this could help, but keep in mind that policy shapes society as much as the opposite. Yeah most social media users are going to roll their eyes and ignore warning messages, but we're out to help people, not necessarily today's people.
When you grow up around the normalization of something like, the "officially sanctioned" knowledge that social media can be dangerous, it gives you something for your brain to connect with when you realize you've been ruminating for hours or days about what someone thinks about you on the internet. It really does help the brain when you can more easily identify a threat.
And more than anything, this would set a powerful precedent in the social view of mental health. Again, when you grow up seeing a thing is normal, you are more readily able to identify the source of the problem when a bad thing is happening to you.