Mental Health Illnesses aren't just "oh I'm depressed, I should kill myself, humph!".

You're either very young, or very ill informed of mental health issues. Either way you should take time to learn about some of the different mental health disorders out there and what kind of hell living with some of them can be before speaking with such absolute conviction on a topic you know little to nothing about.

As for your

It's not necessarily the case that someone with a mental illness is doomed to a life of extreme suffering. That's an ablest mindset, and it does disservice to that community. When someone is in their darkest moment and unsure if things are going to get better, we should be giving them hope, not saying "you're right, you're doomed, but don't worry you can kill yourself".

That's what the medical team is for. They don't just tell you to kill yourself because you had a bad day. Have you ever interacted with a medical team for a long term health issue for any length of time? It is a slow meticulous process of tests and treatments and elimination of options that can take years. Get your head out of your ass and use your brain for fuck sakes.