Often when a popular leader - a pastor, a politician, a teacher - retires from office, somethng slips. What made Joshua such an effective replacement?

Israel served the LORD throughout the lifetime of Joshua and of the elders who outlived him and who had experienced everything the LORD had done for Israel.

~Joshua 24:31

Joshua was an effective replacement for Moses because he not only was taught by Moses as to how to be a great leader, but he carefully watched every bit of Moses' moves so he could become an effective leader