How are those principles not valid today?
This advice would have been good advice then, but today gender roles are much less rigid, and a well-rounded human understands how to balance hard work and purposeful relaxation, how to be strong and persevere and when to lean on others and be vulnerable, and knows the right time to speak up and voice one's grievances.
The grandfather's advice is one formula for a certain type of masculine stoic life, but if you were to follow it too rigidly, you'd miss out a lot of the emotional richness that modern life has to offer.