despite how harmful it is for society as a whole, and especially children
If you don't understand that the motivation is to target kids with ads and influencer content designed to push products, you're not going to solve anything. Kids have to have spaces to communicate with each other in order to develop healthy socialization skills. Locking them in a proverbial box is not healthy, and guess what, we killed off 99% of third spaces that welcome kids.
If social media is banned for under 16’s, then children would have to communicate with normal chat apps.
I feel like you are envisioning "chat apps" to mean "text-only", but chat apps have been multimedia/ multi-modal, and multi-user (i.e. not 1:1 messaging) for a long time now, and can be just as easily infiltrated by the same actors targeting kids on social media.
at some point some systemic problems are better served by systemic solutions
This is not a solution, this is a band-aid that doesn't attack the root cause whatsoever.