That's an easy argument but I'm not sure it's a correct one. The answers to the following questions may be different:

• Is it a good thing on the whole that A exists?
• Now that A already exists, does A want to commit suicide?

for at least the following reasons:

• Other people will be negatively affected (it may be worse to get to know someone and have them commit suicide, than never to encounter them at all).
• People have an inbuilt preference for staying alive, for obvious biological reasons, so someone may go on wanting to live well beyond the point at which each day of life is substantially negative for them. (Just as someone may go on wanting to take heroin well beyond the point at which they'd be happier if they stopped.)

Even without taking into account cognitive biases and externalities, that argument only applies to people who don't believe in afterlife: if you thought that by killing yourself you would go to hell and by enduring shit thirty more years you would go to heaven, you'd think twice before picking the former.