If it was true, then the world would cease to exist after anyone commits suicide.
If he was still needed, Hashem could save him from killing himself.
I heard a story that a famous Rebbetzin saw a construction worker fall from a scaffold. Miraculously, not only did he not die, he was able to walk without medical attention. All the workers were freaked out from seeing the man fall so the foreman let everyone off early. So the guy who feel called out, “Let’s party! Drinks on me at the bar across the street!” And with that, he dashed into the street to be hit by a car which ended his life.
He got a second chance. He was needed for something. He didn’t take his chance and lost his life.
SiDi™