Benjamin declares that he is growing old and that due to age and perhaps infirmity, he is relinquishing the throne to his son Mosiah. He will live for three more years. We cannot tell from what little evidence is in the text what sort of infirmity befell him, but the important point is that where most transitions occur at the death of the king, this one is an abdication. Even when a king was weak, there were advisors to continue the rule, so it is not improbable that this abdication is a special case.
The nature of this speech is to create a newly named people that will be different from the former divided population. Perhaps as a signal of that change, or an emphasis on the newness, there is a change in the king. That ensures that this speech will create a new beginning, symbolically and psychologically.