I personally think Kizaru should've become the fleet admiral. His Devil fruit power plus his speed seems too good of a maych against akainu. When it comes to the 3 admirals, my theory is that its a rock paper scissors scenario in which each admiral has a slight afvantage over another. Akainu's magma beats Aokiji's ice, Aokiji's ice reflects Kizaru's light devil fruit power, and Kizaru's light power and speed,can defeat akainu's magma. I,think it makes alot of sense.