Obviously we don't know the full story, but people make mistakes, underperform, or somehow lose their touch. If it's a pattern and the person obviously just doesn't want to work there anymore, fine.
But on balance, I think it's better to counsel, coach, and develop people over time, regardless of experience or tenure or attitude. Not only do I believe employee churn to be more expensive in the long run (brand reputation, institutional knowledge, handover premium) but I believe it to be ethically wrong.
This attitude has personally served me well. I can't prove it with data, but my heart is whole and I have no regrets about going above and beyond to retain and develop employees.
I'd rather die without guilt and have a packed funeral – than a trillionaire and alone.