The whole point was that Aharon was a human being and the High Priest. As a result he could not replace Moshe Rabbeinu. Moshe was (in the thoughts of the Bnai Yisrael) a direct connection to Hashem, whereas Aharon was merely a human being who was being used as a method of setting up an intermediary to Hashem by performing the worship service. Aharon could no more be the leader of the nation than Moshe could be the High Priest. Their characters were suited to the positions they held, they could not perform the role of the other.
Rabbi Saks points to the pasuk in Tehillim 85:11
There is a wonderful Midrash that bears out this idea, based on the
verse in Psalms 85:11 “Loving-kindness and truth meet together;
righteousness and peace kiss each other.”
Loving-kindness – this refers to Aaron. Truth – this refers to Moses.
Righteousness – this refers to Moses. Peace – this refers to
The Midrash brings proof-texts for each of these identifications, but
we understand them immediately. Moses and Aaron were quite different
in temperament and role. Moses was the man of truth, Aaron of peace.
Without truth, there can be no vision to inspire a nation. But without
internal peace, there is no nation to inspire. Aaron and Moses were
both necessary. Their roles were in creative tension. Yet they worked
side by side, each respecting the distinctive gift of the other.
Rav Hirsch explains on Ki Sisa 32:1
That which Aaron was to make was not to take the place of Hashem, but
of Moses. They presumed that Moses had had an accident and was dead,
and so they wanted Aaron to give them a "Moses" whom they could not
Thus any other human being could not, in their minds, take the place of "Moses" and they would have to create a permanent replacement that could never be lost.