I think that is the only reason why I would ever sell ALL my Bitcoins.
It is better not to sell all your bitcoins, If it is giving you best returns then I would suggest to sell a part of your bitcoins and keep rest for the future, as we all know that future is uncertain and give us some great surprises in the future, so it is better to keep some bitcoins safe in wallet for the future.
I know, and that is why I'm currently holding all my BTC. But, if it was absolutely sure that I could live carefree (as in: not to worry about money) for the rest of my life, I would sell them .