That promise had believability because Joshua saw with his own eyes the reality of the presence of God with Moses. Joshua saw God's glow on Moses face. He heard Moses speak the word and wonderful events transpired. The father of my dreams is supported by the power and presence of God. Often we wonder why we have such limited influence with our children.
True leadership boils down to our ability to influence.
The role of Dad no longer insures influence over our children. As our children grow older they acquire spiritual x-ray vision. They can see whether or not God is with us. The cardinal rule of parenting is this, don't spout it unless you've got it. When you spout what you don't have, you lose your credibility, and then your ability to influence is reduced to zero.