If you can suspend the rights of someone declared mentally ill, the unscrupulous parts of government will be tempted to declare politically inconvenient people mentally ill and strip them of their rights. Even in the United States, this happened during the suffragette movement; those women hunger striking for the vote were sometimes imprisoned in asylums for hysteria.
Even something as simple as detaining a criminal is "carving out an exception to human rights". You're correct that this could be abused, but so can any law. It's possible to mitigate though; that's the entire reason for the concept of an independent judiciary.
Few people would suggest that not having prisons is a better alternative than having them, however unpleasant. Similarly, in a world of deinstitutionalization it seems unlikely that 500k people living on the streets is the best of all possible worlds.
If anything its an example that suggesting to bring back asylums should be considered synonymous to an advocacy where the abuse of detained people is taken into account as an acceptable cost.
But mind you: The last time we implemented detaining people, this was in border control, and resulted in the border patrol trafficking children through abusing the child separation policies at the time. There are still tens of thousands of missing children from this policy, implemented for a scant few years.
I think we have to accept that, unfortunately. Our current situation is untenable, and there really just does not seem to be any way to "fix" someone on the streets with a mental illness without committing them to a mental institution. If you do believe there is another way that will be broadly applicable and successful, please suggest alternatives.
I'll agree that we also need significant structural changes to society (which will lessen the number of people who end up in these bad situations in general), but those changes will take years (if not decades), and the bleeding needs to be stopped in the meantime.
Obviously a problematic/crass analogy, but think of it like getting a hamster before you get a dog. Prove that you can take care of prison 1, and then maybe we can talk about you getting prison 2. But if you can't take care of prison 1, you probably aren't going to be very responsible about prison 2.
And I just don't know if you're ready for prison 2 yet, prison 1 still has a lot of human rights violations in it. Prison 1 doesn't really make feel like we all know how to avoid committing human rights violations against people with mental health issues, it really makes me feel the opposite.
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I don't know, I genuinely don't mean to be snarky about it, just... it's a little weird and uncomfortable to be sitting in the realm of theories while discussing a system that actually exists in the real world and that horribly hurts people on the regular, and to say, "we'll just expand that and have the same standards." People with mental health issues already get incarcerated and they're treated horribly, and we're glossing over that when we talk about incarcerating more people.
This is basically the argument of the pro-gun-control folk. The counter-argument I keep hearing is that rights are sacrosanct, any concomitant negatives are the price of liberty.