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?