1) They may or may not. Very realistically may not. So what’s the basis for the argument? Because it’s more enjoyable?
2) We can and will tell people to do something that may have negative effects on their mental health to measurably save someone else’s life. We do it all the time. Like, always.
Every single law enacted ever can be argued that it “adversely affects someone’s mental health”. My point is, you don’t even know that there *will* be more mental health issues from lockdown than from just having the virus, and I can make a coherent argument against it, but we DO know that more people will die (and the person being told to lock down is one of those at risk too. Plus their family. Who’s lives they certainly don’t have a right to risk).
Now, we can fight about seatbelts and assisted euthanasia and abortion and all sorts of libertarian beliefs if you want, but don’t get all indignant if we force people to *perhaps* sacrifice some comforts/mental health to *definitely* save many lives. Sorry.
Mental health repercussions are very real. I don't care if you can't quantify them, they are real.
If you want evidence of it you can start here
https://thehill.com/changing-america/well-being/mental-health/491585-the-psychological-effects-of-coronavirus-quarantine and this was not a national indefinite lockdown, it was a quarantine for select people I assume for no more than 2-3 during the SARS outbreak.
Anytime something will adversely affect someone else we should take that into serious consideration. The fact that you keep making light of it shows that you're seriously underestimating its impact.