I'm genuinely curious, is that a widely accepted position on suicide and mental stability? I can think of a number of situations where suicide might be preferable to continued existence.
I think it may be widely accepted, but I also think it's nonsense. As you allude to, someone with a terminal and painful disease could have an entirely rational desire to commit suicide based on a sensible projection of the future as containing a persistent unacceptable level of suffering.
I think we should only invoke "mental instability" when someone's projection of their future level of suffering is clearly mistaken.
I think it's theoretically possible for someone to arrive at the decision to end their own life via careful evaluation, but it's quite unlikely (and at this point, the only real acknowledgement of it in the psychiatric field is in the case of terminally ill patients, when the calculus is pretty clear).
Regardless, the vast majority of suicides are the result of a combination of depression and loss of inhibition, both of which are psychiatric symptoms that can be brought on or exacerbated by severe stress. The assumption can and should be that this is the case, not that an individual who was self-admittedly under huge amounts of personal stress performed a rational evaluation of their own future prospects in life and determined that it wasn't worth living.
Suicidal thinking is almost always a mental health problem or the result of a mental health problem.
You say that there are some situations where suicide is a rational choice. But even in these cases it's not that clear.
While I respect the right to chose death, and support changing of law to allow people to seek medically assisted suicide, I am worried that people with undiagnosed depression who would respond to treatment are allowed to die.
I think we should only invoke "mental instability" when someone's projection of their future level of suffering is clearly mistaken.