One day they too will reach a ripe old age. Whereas by then they will have most likely forgotten their dereliction of duty towards their own parents/grandparents, Hashem keeps a very precise accounting of our every deed and repays in kind.
I don’t mean to sound morbid, but that’s the reality.
Just telling it like it is
No argument there. However, our benevolent Creator does not seek vengeance but rather waits for us to do the right thing and to constantly strive to grow in ruchnius.
That said, parents who drop the ball and fail to do what’s right unwittingly pass those failings on to their children — who tend to emulate the actions (or inactions) of their parents.