Mechanical equipment like pumps in active use don’t last anywhere close to 50 years and need to be overhauled or replaced several times over that 50 year lifespan. You can find videos of turbines being replaced which is incredibly expensive. In the end you don’t get a clear this is the final date you can operate limit just increasing costs every year.

The ~fifty year lifespan is in part based on physical corrosion of pipes running through concrete there really isn’t a way to economically replace them all that costs less than simply building a new power plant. But even here not everything fails on the same day so there is some wiggle room.