You know what you don't get in an isolation cell ? An optimistic outlook on life.
Even in those circumstances patients often figure out how to commit suicide anyway with the very limited tools they have there. There's plenty of examples of patients that figured it out after 2 months or so. Of course there's also examples people who haven't figured it out after years, but I doubt anyone who's seen that considers it a good outcome.
You have to realize that you're choosing between 2 alternatives. You cannot just choose against something. What I've described here is the only alternative, and yet I feel like the vast majority of people would give these people an alternative, even if it was suicide. I don't think you'd really consider 10 years of this a better alternative to suicide if you were to go and see one of those patients.
And of course, there is the alternative "provided": get the staff angry enough (damage a few of them permanently for example, systematically attack them psychologically and pester them), and there is a point where they will throw you out (will make things worse in the short term of course). That's another thing that can't be prevented.
And I get it. There are other problems. Who judges these people ? Because obviously there's problems there. Who decides if a request is sincere and not forced ? Who executes such requests ?
But still allowing suicide is not the worst thing to do.
Are we running out of people? Can we make life bearable for them? No, and alas not really. Sure, we offer drugs, and counseling, and whatever, but sometimes those amount to nothing, because you can't give them what they need/want - kinship, family, love, a loving mother/father, a better childhood, etc.
I'm not saying life is not precious. I'm saying, that exactly because life is precious because it allows you to experience the journeys, pursue happiness, find joy, retrospect, and go out knowing it was precious. But if none of these happen, if none of these are available, if you can't give any of these to these people, then their life is not a life.
And in particular when it comes to young adults there is a significant possibility that treatment resistant mental illness improves or sometimes vanishes, often in the late twenties or early 30s. In the case of a 17 year old girl I think precaution dictates that we stay on the side of treatment. I think to end the life of a minor without terminal illness is very hard to justify.
And I thankfully never suffered from depression, but have a family member who did. She came very close to ending her own life, and explained to us repeatedly that she did not want to live. When it took a turn for the worse we had her hospitalized against her will, and now several years later she is thankful for the fact that we did. And I think I'd feel the same way. Mental illness can change the perception of people so much, it really seems to hard to me to take these wishes just at face value or see them as personal choice.