In 14:13 it says "וישמעו מצרים" and Rashi explains that they would then say that "Hashem was able to beat us in war and take the Jews out, but He is not strong enough to beat the יושבי האארץ".
Moshe was calling Hashem out and warning Him that if he kills them out, if He does not let them into the land, then He will have caused a chillul Hashem of sorts.
I would like to suggest that this might be why Moshe later gets punished by not being allowed into Israel upon striking the rock.
Over there it is explained that Moshe did not really do anything wrong, per se. Rather, he could have effected a greater kiddush Hashem by talking to the rock, but he struck the rock instead.
But why is that so bad? So he caused only a smaller kiddush Hashem? For that he deserves such a harsh punishment?
I would suggest that it is a sense of tit for tat. Mida k'negged mida. He got the punishment over there for not effecting a greater Kiddush Hashem, because over here by the spies he used that argument against Hashem to protect the jews. He said "Don't destroy the jews because the Egyptians will say You were not strong enough." By destroying the Jews, You will be preventing the greater Kiddush Hashem of taking them into the land and everyone will recognize Your strength..
Because Moshe used that argument here, so later when he prevented the greater kiddush Hashem, he had to be punished via the argument he introduced here - by not entering the land.