Loneliness and a lifetime of sadness has been shown to have greater risks. Is cutting out social connections the answer? Because that's what ends up happening.
Please present evidence that denying teens access to social media causes them to have literally no meaningful social connections. I do expect social interactions to suffer for a teen in that situation. But I find it hard to believe that it causes complete isolation and (gimme a break) a "lifetime of sadness".
The parent didn't say "literally no meaningful social connections". You're asking for proof of a point that wasn't made.
Sadness, loneliness, isolation, these are not things that you can measure with such binary precision. They're emotions, and they vary, person-to-person, and moment-to-moment. Is it not plausible that for some people, 'social interactions suffering', even if not completely removed from social connections, is enough to make them feel a profound level of sadness or loneliness or isolation?