What is the reason משה רבינו asked specifically for איש – a person?  Could the future leader of כלל ישראל be anything other than a person?

It’s brought in various places that if ח"ו there is a משחית  - destruction – it doesn’t differentiate between צדקים  and רשעים. 

We can say that precisely because of this, צדיקים are a protection for their generation.   If ח"ו  for whatever reason ה' has reason to allow a משחית to come down to this world,  He takes into consideration that צדיקים might also be damaged. He thus rescinds the decree in the merit of the צדיקים so that no damage come to them.  In this way, the entire world is saved, in the merit of these צדיקים.

This protection refers to those צדיקים who have a connection of some kind to this world.   For them, ה' will change His decree regarding the world, in their merit.   There are צדיקים on an even higher level, almost on the level of מלאכים, who have no connection to this world.  For these צדיקים, what happens in the world around them has no effect.   For צדיקים such as these, ה' sometimes doesn’t rescind a decree, since it has no effect on the צדיק.

Clearly, the leader to replace משה רבינו would have to be a great צדיק.  What משה רבינו requested of ה' was that this leader be also איש – a person with a connection to this world, and in his merit, ה' rescind harmful decrees.  If this leader were like a מלאך, he might have less capability to do so.

Tzidkas Yosef, Parshas Pinchas