It doesn't seem too wild to use irrational belief in something (faith) to make oneself happier through tough times.
It does seem wild to spend your final days in a pit alone until you starve/dehydrate to death painfully.
These monks aren’t impulsively killing themselves, they seem to be towards the end of their lives anyway and are choosing how they want to go out. I don’t see what’s wrong with that. I might prefer to be in hospice hopped up on drugs when I pass, but I won’t claim to be morally superior for it.
Religion is basically always a socially acceptable mental illness that we not only tolerated but encouraged because it has been a part of civilization since before we formed the basic premises for what constitutes a civilization.
That doesn’t make it an illness, unless it produces massively negative outcomes.