These calculators have a large error built into them by default. The idea is that it is better for many people to spend too much on a power supply than a few people go through the troubles associated with a weak a power supply.
Your concerns about the life of the power supply are valid. If the power supply degrades, it could provide unsteady voltage. I would do nothing for now, but as soon as you notice stability problems, replace it.
Your concerns about the life of the power supply are valid. If the power supply degrades, it could provide unsteady voltage. I would do nothing for now, but as soon as you notice stability problems, replace it.